No Biochemical Barriers

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"R2: "Obviously the air is highly caustic and poisonous."

Obi-Wan: "Apparently not.""

- Darths and Droids

In Speculative Fiction, it is far too often the case that writers do not take into account the fact that the differences between terrestrial species and their alien counterparts would run deeper than appearance alone. Given that alien species would have evolved in environments vastly different to terrestrial ones, contending with different atmospheric conditions, different levels of background and cosmic radiation, different soil conditions, different pathogens and parasites, and so on, it is patently unlikely that any aspect of their biology would be directly compatible with their terrestrial counterparts. Aliens in speculative fiction are usually depicted less as properly alien creatures and more like humans from another country, and alien animals are merely exotic species from a different climate.

This, then, means that you will often find one or more of the following in speculative fiction:


 * Half Human Hybrid - the most blatant offense against conventional biological wisdom. See that entry for examples.
 * Hot Skitty On Wailord Action - The same lack-of-mechanism tossed into Mon series.
 * Alien Food Is Edible - Humans and aliens are able to share food and drink. As an extension of this, humans or aliens shown as being able to derive sustenance from utterly alien animal or plant life. (Aliens eating humans also falls into this category.)
 * Cross-Species Disease - Humans or aliens affected by alien pathogens or parasites that should not have had time to adapt to their physiology. This in spite of the fact that on Earth there are huge numbers of viruses, bacteria, and parasites that only affect a single species or a few related ones, and that so-called "zoonoses" have generally only been a big deal amongst livestock, which we've intentionally cultivated in close proximity for ages.
 * All Atmospheres Are Equal - While species that breathe something other than oxygen are sometimes seen wearing protective gear when moving in oxygen-based atmospheres, species that are capable of breathing an oxygen-rich atmosphere are never shown to be hampered or even affected at all by the other gases (nitrogen, for example) that constitute the air familiar to humans. Nor do they suffer any problems related to the air pressure -- presumably, All Planets Are Earthlike.

Some of these are more plausible than others, although not by much. While there is reason to believe that Earth's atmosphere carries life because it is particularly suited to life, this is basically a confirmation bias -- we consider it suited to life because it is suited to our life. This position is called the anthropic principle.

Similarly, some people claim that most life-forms in the universe would be fundamentally compatible with each other because they would be formed from the same basic chemicals. For example, despite all earthly life stemming from a common ancestor, a significant amount of the plant life on earth is unsuitable to downright deadly for human consumption, while animals that do eat plants generally are able to only due to a bacterial symbiotic relationship with the only organisms who can. Considering that life on other planets wouldn't even necessarily be carbon-based, this is a very naive position. Also, arsenic is one of those "same basic chemicals" as well.

This is one of the problems to watch out for in How to Invade An Alien Planet.

Anime

 * Dita Liebely, the female protagonist of Vandread, samples the food from Hibiki Tokai's home planet, but she finds it revolting and inedible. Believing the opposite must be true for Hibiki, she feeds him food from her home world, which he found to be delicious. Following this, Dita has little problem spending time with Hibiki on the condition that she feed him. Of course, although Dita considers Hibiki to be an alien lifeform, most outsiders would point out that they are the same species, just different genders.
 * Aliens love Pocky.

Comic Books

 * Tenzil Kim and his fellow Bismollians from The Legion of Super Heroes can eat anything, except certain materials he specifically can't use his power against.
 * In Tom Strong #16, it's established that while unaltered humans can eat Devil's Footstool food, the opposite is not true. (With a few exceptions, such as coffee.)
 * In the backmatter for the second volume of League of Extraordinary Gentlemen, this is a problem for Alice of all people after she returns from Wonderland through the looking glass. Her whole body is reversed left-to-right, so her hair part is on the wrong side - as is her heart. Then, she mysteriously dies of malnutrition, despite eating normally for a while. It's not explained in the text, as Victorian science wouldn't be able to figure it out, but the science-minded reader might conclude that the reversal extended all the way down to the molecular level, and that Alice's amino acids were backwards compared to those in her food.
 * Green Lantern. The human Green Lanterns have problems getting good food at the Oan cafeteria. The chef just isn't that skilled.
 * In Transmetropolitan the Vilnius and late-stage transients (humans turning into Vilnius) can't eat human food, which helps influence the transient secession movement in the first volume.

Film

 * Many films where aliens, particularly sentient ones, eat humans (humans being eaten by alien beasts can be explained by the alien eating things that act like prey without knowing that it's bad for the alien's health).
 * The Men in Black films and cartoons imply that all alien immigrants and visitors to earth can eat (or appear to eat, in the case of the Mobile Suit Human-wearing Arquelliens) human food, although most of them need to in order to live as ordinary humans.
 * The aliens in District 9 eat a lot of beef and pork, and have a craving for tinned cat food. Though, they also chew tires as if they were bubblegum, so their biology is somewhat different.
 * In Alien Nation, the Newcomers are perfectly capable of eating earth food, with some variants. They prefer more exotic meats (such as beaver), which they eat raw as they can't absorb nutrients otherwise; they get drunk off spoiled milk; and the drug that drives the plot of the film . They also dissolve in seawater and use arsenic flakes as a food seasoning. Justified in that they are a genetically modified life form.
 * Averted in Avatar, in which the planet's atmosphere is deadly to humans.
 * Averted for comic effect in Galaxy Quest, when the actor that the aliens think is an alien is treated to food from his "native" planet... seaweed-like stuff and live bugs in a bowl of water. It also has the Only Sane Man, Guy, pointing out that going out on the alien planet without checking its atmosphere first is insane. They do it anyway (and survive).

Literature
"Wes: "What's that?"
 * ~The Hitchhiker's Guide To The Galaxy~ has approximately 85% of sentient races having a drink called a "gin and tonic" (or "jynnan tonnyx", or "jinond-o-nicks", or "gee-N'N-T'N-ix", or "chinanto-mnigs", or "tzjin-anthony-ks"...). However, all these drinks are radically different, ranging from a gin and tonic to ordinary water at slightly above room temperature to a drink that supposedly kills cows at a hundred paces. Yet they all have the same name, and were all named such before the races had discovered space travel. This may also apply to Ouisghian Zodahs.
 * Also specifically averted, as Ford tells Arthur that things like nuts and berries native to alien planets may well kill you. (Or they may not, but there's normally no way to tell except to try them, so only try them when you're at the point of dying if you DON'T find something edible.)
 * Averted in 2061 by Arthur C Clarke. A spaceship lands on Europa, one of the crew is killed, and the corpse is eaten by a large water creature. Said creature promptly dies, because the biochemical differences means that human flesh is poisonous to it.
 * Averted in the My Teacher is an Alien series: when a human on a space ship full of thousands of species goes to the dining hall, a couple of crewmembers analyze his biochemistry and hunt through the menu/database for something that they're almost certain will not make him vomit or drop dead.
 * Averted in Larry Niven's Ring World: the protagonist mentions that he once survived an attack by an alien beast because it stopped to swallow the chunk of him it had already ripped off and promptly curled up and died, poisoned by his biochemistry. And Justified in the case of those lifeforms that evolved from abandoned Slaver foodplanets, like Earth and Kzin.
 * Touched on in the Star Wars Expanded Universe; though most species apparently have comparable digestive systems, a few unusual species, such as the Gand, enjoy foods that would either be indigestible or outright poisonous to others.
 * In another example, an Arcona, snacking on ammonia crystals, covers his plate when a human reaches for the salt, explaining that sodium chloride is a dangerously addictive drug for his species.
 * Another example is that there is apparently only one human bartender on Tatooine, due to the insanely large amount of information about drinks that needs to be memorized so as to not kill customers with the wrong drinks. In another novel Han mentions that the bartender at the bar better not be watering down the drinks, since the species he's serving is allergic to water! Said bartender was also attempting to develop a perfect drink for Jabba. A single drop of the prepared liquor was enough to leave him convulsing on the floor.
 * After Tycho kills a cave creature in the X Wing Series comics:

Tycho: "Might be dinner."

Wes: "Are you nuts? What if it's toxic?"

Tycho: "Let's cook it and see how it smells.""

""How was I to know your kind can't eat sweetweed?"
 * Turns out that it's a ronk. Female ronks are physically nearly identical to males, but their flesh is highly toxic when eaten. Fortunately, Tycho killed a male.
 * Hutts are also apparently indigestible, even to the Sarlacc, but Hutts can seemingly eat anything; in a story about Jabba before he was killed, it was revealed that someone was attempting to poison his food. The problem for the investigators? Over half of what Jabba ate was poisonous to most other species.
 * An aversion comes up as a Noodle Incident in Death Star.

"You could have looked it up. You plan to date outside your species, it's on you to know what's poison and what's not."

"You're never going to let me forget it, are you?"

"Not a chance, Green-Eyes.""

"Troll: I knows what they say about us trolls, but it not true! We made of rock. We don't eat humans!
 * Averted in Fallen Dragon by Peter F. Hamilton, where alien ecosystems are completely incompatible with human biochemistry -- meaning the first step in colonizing a new world is to kill all the native flora.
 * Played straight in his The Nights Dawn Trilogy, where many exotic foodstuffs are imported from alien planets, and earth plants are genetically tweaked to grow on other worlds; the best alcoholic beverage in the Human Confederation is made from the water accumulated in the flowers of Norfolk roses. A short tale describes how some of the first scientists to land there, while inspecting the local flora, tasted said water. Norfolk was cleared for colonization days later as a result.
 * Apparently, anything in the universe they can get a hoof into is edible to the Andalites. They absorb nutrients, not consume them, so presumably they avoid anything they don't need/want. This deprives them of a sense of taste though, so Hilarity Ensues whenever they morph into humans. Ax also complains at one point about accidentally eating snails when he grazes since as a herbivores it's probably quite disturbing to him that he just killed and ate an alien creature.
 * Similarly, Taxxons can apparently eat just about anything (or anyone). Also, Hork-Bajir are capable of eating the bark of Earth's trees.
 * Averted in John Ringo and David Weber's Prince Roger/Empire of Man series, where, while stuck on the alien planet, a good deal of effort is spent trying to find foods close enough that their nanites can adapt them to provide what they need.
 * Weber's In Death Ground and The Shiva Option have a Bug War against aliens who eat all sapient species, and even breed them as food sources on conquered planets because everything made of meat is food to them.
 * Also averted in John Ringo and Travis S. Taylor's Into the Looking Glass series, where the alien food is used as a diet plan, because, while filling, it has no nutritional value, and thus doesn't provide energy in a form human biochemistry can use.
 * Used in that anyone from one of the four biological patterns (called Green, Blue, Red and Orange based on the version of chlorophyll found) can eat anything of that pattern. However, even same-pattern foods may or may not be nutritious, or even safe- there's just a better chance of it being digestible.
 * Also used and averted in another John Ringo series, The Legacy of the Aldenata, the Posleen are a ravening horde of Extreme Omnivores who can and will eat anything, including each other, but are pretty much immune to infection and disease of any kind. However, its justified by the fact that the Aldenata were trying to modify them into a less violent species. Oops...
 * Lampshaded by a member of a crustacean-like species trying to convince humans to abandon its attempts at developing biological and chemical weapons against the Posleen. Obviously, when the survival of one's species is at stake, ethical considerations are a hard sell, but once it was pointed out that any human exploiting its resemblance to a terran lobster would grow sick if not die from the alien biochemestry, while the Posleen are unhindered by such considerations, it became clear that any attempt at creating a biochemical weapon that would 'zap' Posleen but leave humans and their allies un-harmed was a fool's errand.
 * This gets subverted in John Ringo's latest book, Live Free Or Die, where maple syrup is an incredibly potent alchohol-like substance for one of the alien races.
 * Averting this trope is the whole premise of Larry Niven's Destiny's Road, where human colonists populate a biochemically incompatible planet. People gain no sustenance from native life (and vice versa). They live on transplanted Earth animals and plants, but something different about the place means essential nutrients are missing, so humans must regularly consume something called "speckles" or go insane and die. The native life does not contain fat, or (key plot point) potassium (with the exception of a few plants). You CAN survive on the native life forms (at least for a lot longer than by starving) but you'll gradually lose all body fat, so makeup and source of "speckles" is a major plot point. This is used by a character trying to lose weight, as you feel full, but your system can't process the food. You excrete everything and keep nothing--rapid weight loss happens then.
 * Somewhat touched upon by Harry Turtledove's World War Alternate History series. The Race's biology and biochemistry are similar enough that they can live on Earth and eat some Earth biomatter with pretty much no problems. However, what would be slightly chilly to humans is beastly cold to them (apparently, their home planet is so hot that ice only exists in laboratories) and household spice ginger is so addictive to them it becomes their version of cocaine. It also triggers "heat" in their females.
 * Averted in Eric Flint's Mother of Demons, in which humans can eat nothing on the planet Ishtar but "childfeed," which is semidigested plant material regurgitated by large herbivorous lifeforms (normally to feed their offspring). Additionally, it turns out that
 * In Chasm City by Alastair Reynolds, one human is tortured by being fed to a hamadryad (a giant snake-like alien) which swallows him whole and promptly dies due to terran proteins being lethally toxic to it. The victim is cut out of the creature's stomach, still alive if a little unhinged by the experience.
 * Averted in The Mote in Gods Eye. After a good deal of biochemical research, the Moties eventually come up with some local foods that the humans can eat. Moties find coffee about as lovely as torture, but they do like hot chocolate.
 * Don't forget the drop of machine oil in the cup!
 * Specifically mentioned in David Gerrold's The War Against the Chtorr novels: the invading Chtorrans have absolutely no problem chowing down on Earth lifeforms, leading human scientists to speculate that the attackers were deliberately created/bred to be compatible. (Whether this is true or not is yet to be revealed.)
 * In the 1992 novel Murasaki, written by several well-known science fiction authors, this trope is averted in an almost sad way. Humans discover the planet Murasaki and mount an expedition. The local life is almost completely poisonous to humans because they use a wider variety of amino acids. The natives, in an effort to make friends with the humans, find one life form that has some parts that are edible to us and present them in a feast. They are delicious. But they are also now extinct. Every last of the animals was harvested in order to provide the feast.
 * Subverted in Terry Pratchett's Strata where an important plot point is that the carnivorous Silver has a different biological makeup that makes it impossible for her to eat meat from a different planet. Being a predator, she will however turn into a ravenous badass killing beast which will still kill everyone in sight if her hunger gets the better of her.
 * Also subverted in Pratchett's Discworld novel "Moving Pictures".

Other Troll: Swallow. We don't swallow."


 * Used somewhat in Allen Steele's Coyote. A colony ship of humans going to a new, alien world doesn't know, while in orbit, if the planet has the right kind of soil(dextro versus levo amino acids may have been involved) to grow Earth crops; the colonists couldn't have survived on native food from the wrong kind of soil. They were lucky.
 * Tully, the sole human in C.J. Cherryh's Chanur Saga, has no problem with Hani food.
 * In the Foreigner series humans must be very careful as to what atevi food they eat. Many "teas" and seasonings are deadly to them. The atevi can eat all human foods without problem, though human food is lacking in essential atevi vitamins, so an atevi can't survive on only human food.
 * It is mentioned in a Dragonriders of Pern novel that humans cannot eat any of the native Pern plants and can only eat the plants originally brought from Earth, but they they can eat the native animals.
 * McCaffrey also averted this trope in Dinosaur Planet, in which the exploration team heavily tests any local organism they consider using for food. Also,
 * Both played straight and averted in the Humanx Commonwealth universe of novels by Alan Dean Foster. The vast majority of species in the galaxy seem to be able to exist just fine on the same basic foodstuffs and in similar atmospheric conditions. To be sure, analysis is required to ensure compatibility before attempting to eat strange foods, and there are some cases where things are explicitly pointed out as being toxic to certain species, but the trope is obeyed in a general sense.
 * Averted in David Brin's Uplift novels. A food eaten by one species is likely to trigger an allergic reaction in another. One planet has so thoroughly incorporated heavy metals that dolphins who swim in its oceans need chelation afterward. In an unpublished manuscript of what would become the Jijo trilogy, a Hoon goes into a delirium when a human child gives him peanut butter.
 * Played both ways in the Vorkosigan Saga by Lois McMaster Bujold. The wildlife on the newly discovered earthlike planet Sergyar is edible, but has to be well cooked because native parasites might find humans edible as well. On the other hand, the native flora of the world of Barrayar is hostile to its human inhabitants. Most of the plant life is some shade of red or brown, is completely inedible (and often poisonous to humans), and produce enough environmental toxins that mutation can be a real problem on the planet. Especially during a centuries-long period of isolation from the galactic community leading to the loss of modern medicine. Barrayarans developed a very Sparta-like attitude towards birth defects and deformities as a result.
 * Averted in Courtship Rite, by Donald Kingsbury, where humans on an alien planet can eat nothing but the ""sacred eight" organisms which survived their arrival. There is some talk of being able to process a few of the native plants to make them non-toxic, and of carefully selecting the edible bits of other native species. Some groups of humans have deliberately selected themselves for greater resistance to the poisons in the native organisms. The available foods include honey bees (I think the only animal) and wheat. Cannibalism is part of the culture, with baby's tongue being a delicacy!
 * Averted beautifully several times in the Ender's Game Speaker for the Dead trilogy.
 * A human colony on an alien world has to grow their own crops because alien plant species are either inedible or nutritionally useless. They are also trying to genetically alter the local plant life to make it a food source for humans.
 * The humans have to genetically engineer their own wheat crops in order for the the natives to be able to eat it.
 * The locals on the colony world can eat one of the native grasses and it acts as a pain suppressant. Several major plot points revolve around the fact that.
 * The planet is ALSO home to the most deadly virus-like diseases in the known universe (it has a nasty habit of "ungluing" DNA, splitting the helix into two useless strands and killing the cells in a horrifying and painful manner). It turns out that not only is this disease rapidly "learning" to overcome what protective measures the human colonists have devised  meaning any attempt to simply eradicate it would essentially be
 * In Neal Stephenson's Anathem, it is shown that . This results in the food not being toxic, but being totally indigestible, resulting in it passing through a digestive tract unchanged. Similarly, alien pathogens are explicitly mentioned as being no threat, at least until they've had a chance to evolve a little.
 * In Roger Zelazny's Doorways in The Sand, the main character stays on Earth, but gets run through a dimensional inverter, resulting in him being mirror-flipped. The first time he tries to drink beer he realizes that all molecules are now backwards to his physiology and he had better stick with food and drinks that have symmetrical molecules, or else he'll poison himself.
 * Averted in the Corean Chronicles. The world was colonized and mildly terraformed to allow humanity to survive. The majority of the surviving fauna and flora is completely alien to their biology, consisting largely of Life Energy interwoven with the natural elements of the land. The meat of nightsheep, the only known fleshy creature able to subsist on native flora, is poisonous to humans.
 * Unusually for Star Trek, it's averted in Articles of the Federation. The Trinni/ek food fleer/ok has to be removed from the menu during preparations for a state dinner welcoming the Trinni/ek Speaker. It's determined to be poisonous to Bajorans, Betazoids, Humans, Trills, and Vulcans, and causes an allergic reaction in Tellarites.
 * The Apicians in the Cordwainer Smith short story "From Gustible's Planet" are capable of eating Earth food (and think it's delicious). This is specifically said to be unusual, though.
 * In addition to the disease issue, this is also brought up in the Sector General novels. Specifically, the cafeteria at the hospital has a very wide selection of foods on the menu, with notations as to what foods can be safely eaten by what species. Occasional problems come when a diagnostician comes in for lunch after having a tape personality active for awhile, and as a result has to force himself to pick a meal that is suitable for his actual body rather than what his alternate personality thinks he is, and then force himself to eat what looks to be a highly unappetizing substance from the tape personality's perspective.
 * The food of one planet is inedible to a native of another planet in The Madness Season. In addition, the main character, a shapeshifter is only able to transform into creatures native to the planet Earth.
 * This trope goes all the way back to the first Space Opera, The Skylark of Space. Though the main characters are initially wary of eating Osnomian food, Seaton gives some Earth spices (as well as cigarettes) to an Osnomian without being at all worried for the alien's health.

Live Action TV

 * Babylon 5 features several examples of characters consuming food from alien societies. Sometimes there's no problem, other times... Well, this is what happens when Centauri try human fast food: "It tastes so good going down. Coming back up, it's not so good." And just about everybody can (and does) eat spoo. Even pak'ma'ra, who are carrion-eaters with a strict religious-based dietary code.
 * At one point G'Kar claims that every race has independently created their own version of Swedish Meatballs. According to the card game, even the Vorlons have one and it's apparently a sentient lifeform. Very much Played for Laughs.
 * When Londo Mollari asked what an item a salesman was offering, he said "Candy, only for carbon based life-forms who can metabolize sugar, otherwise it's decorative."
 * Alcohol appears to have exactly the same effects on Centauri that it does on humans - and, it's implied, on Narns and possibly other aliens as well. On the other hand, Minbari react quite badly to booze and become feverish and homicidally violent.
 * In the novel To Dream In The City Of Sorrows, Sinclair has to drink a certain Minbari beverage as part of the ritual to become Entil'Zha (Head of the Rangers), and is nearly killed by it. The Minbari knew this would happen, which is why that part of the ritual was delayed as long as possible, he only drank one sip, and they had a medical team standing by to treat him the moment the ritual ended.
 * The Doctor from Doctor Who is apparently some kind of super-omnivore, with the exception of aspirin (and presumably other salicylates, like wintergreen Lifesavers) which is very toxic to Gallifreyans. However, the effects can be counteracted by consuming significant amounts of triglycerides (i.e. fat, in one specific case cocoa butter). It gets even worse when the Slitheen (who are calcium-based life) are shown eating regular human food so as to keep up appearance and thus not blow their cover. Must have been a bugger avoiding vinegar all the time, maybe they should've started their campaign by invading France.
 * A mild aversion in Stargate SG-1. In the Season 4 episode "Small Victories", Sam is taken to the Asgard homeworld to help them defend against the Replicators. When she asks for food, Thor beams in some colored food bits and recommends the yellow one. Carter finds it disgusting. Thor presumably knew she could eat it just fine, but probably didn't take taste buds into account.
 * Star Trek is a flagrant and persistent violator of this one. Humans who like Earth dishes are in a distinct minority. In fact, almost any foodstuff mentioned is prefixed with an alien adjective ("Rigellian cheese", "Centauri trifle") while Klingon food gets its own vocabulary and a legion of human devotees. Practically the entire crew of Deep Space Nine seems to subsist on Raktajino (Klingon coffee). At least some examples may be cases in which Earth-derived foodstuffs are merely prepared using techniques introduced from other worlds.
 * It's subverted in one episode of The Next Generation, where Doctor Pulaski gives herself an injection before drinking some of Worf's tea, and the Klingon tea is also poisonous (though not 100% fatal) to Klingons, as the point of the tea ritual is that "Death is an experience best shared". Most Klingon food is just fine for humans. The barrier? Most of it is still moving. Humans have a problem with that.
 * It was also subverted in one of the paperback novels, where Riker has to go to sickbay after having lunch with Worf and accidentally eating some items which were explicitly listed as indigestible to humans.
 * Episode "The Way To Eden" in TOS averted it. The plant life of Planet Eden was toxic and corrosive.
 * Used as a plot device in one episode, "Allegiance". When Picard and a group of three different aliens are captured for a 'rat in the box' type experiment, one of the aliens cannot eat the food provided. Given that he is a predator of other species and very violent, this becomes a reason to escape.
 * And there was an episode of Enterprise featuring a human crewmember stranded on a hostile planet with an alien. They tried sharing a canteen, which didn't work so well. Of course, the alien also had some kind of spittle that could rapidly heal wounds on human skin.
 * Another episode of TNG had Picard and Crusher stranded on an alien planet. Crusher was injured, and had Picard pick a random herb for its medicinal properties, which she could determine because its juices turned his skin yellow, and it tasted bitter (yes, the DOCTOR had Picard taste it before she knew what it was). This could have been justified as part of her medical training, except she says she knew of it because it was a folk remedy in her native Scotland.
 * Deep Space Nine touched on this at least once. Sisko, annoyed by an assassination attempt made on the station, sits down with Gul Dukat and Weyoun (the official representative of the Dominion) with the bottle of poisoned kanar (Cardassian booze). As Sisko and Dukat argue, Weyoun knocks back a glass of the stuff, and exclaims, "Oh, that is quite toxic, isn't it?" Turns out Vorta are immune to most known poisons. This is probably due to the fact that the Founders of the Dominion pretty much genetically constructed the Vorta from the ground up to be their perfect servants.
 * Averted in Star Trek Voyager. When Neelix and another person beam down to look for what the ship scans say are edible flora despite slightly harsh atmosphere (only slightly toxic, if a human (or Neelix) are exposed to it for too long could cause a rash and then after about three hours get seriously ill), they instead find nothing. The atmosphere actually also has amino acids and other nutrients in it that the local sentient but primitive live on, and it's slightly toxic to other species.
 * Played amusingly straight in an early episode where a large portion of the crew beams down to a planet to forage for food. One crewman locates what appears to be an apple and is about ready to bite into it before Neelix explains that it's highly toxic and will cause all sorts of unpleasant reactions. This despite the fact that by this point Neelix has known of the existence of humanity for maybe a month and definitely doesn't have the medical knowledge required to know what would happen to human physiology if they ingest the fruit. He then goes on to recommend a less tasty but much more suitable plant instead...
 * In TNG, it was revealed that the transporters have a "biofilter" that is capable of removing pathogens (but to look at it sardonically, it basically doesn’t ever work any time an episode is about bringing a plague onto the ship).

Tabletop RPG

 * Partially justified in Traveller, where the Vilani (a race of Transplanted Humans brought to another planet in ancient times by meddling Precursors) have a culture shaped by the immediate incompatibility with their environment; one of their governing castes grew out of those with the necessary skills to chemically reprocess alien foodstuffs into human-suitable nutrition. One of the disadvantages of this history was that their biotechnology suffered from lack of compatible pathogens, and once they went to war with the Terran Federation many died of disease.
 * Averted in Centauri Knights. The surviving Centaurian ecosystems on the space colonies are home to incompatible foods and creatures, but a biomod is available to change a character over from Earth food to Centauri food permanently, with no mix-and-match. The planet itself has no ecosystem left, as it was wiped clean to the bedrock by Nanomachines thousands of years before, and thus humanity is able to begin terraforming it without contamination.
 * In a Played for Laughs subversion, school dietitians in Teenagers From Outer Space react to the arrival of thousands of new and weird alien species in a unique way: they keep serving the same stuff they served all along. After all, it wasn't quite meant for human consumption anyway.
 * Averted in a sourcebook for Werewolf: The Forsaken dealing with werewolf physiology and behavior. The book makes it very clear that becoming all fuzzy when you've got chocolate, caffeine or drugs in your system can mess you up something fierce (as chocolate and caffeine are trouble for canines, and... well, the drugs should be fairly obvious, shouldn't they?).
 * "Alien Wars", a Military SF setting for Star Hero, features the Xenovore race, which can metabolize any meat regardless of which planet it comes from. Sentients are the tastiest, but they'll eat nonsentients if that's what's available.
 * The number of half-X, half-Y creatures in Dungeons and Dragons borders on mind-boggling. Depending on the edition and setting, you could encounter a half-elf, half-orc, half-dragon, half-demon, half-devil, half-angel, half-robotic-thingy, half-undead, half-eldritch abomination, half-god, half-halfling, half-giant, half-fey, half-chaos frog thing, half-elemental, half-djinn, half-dwarf, half-furry, half-golem, half-mutated egg-laying cousin of humanity, half...oh, you get the idea. Most of them can eat and drink the same things (if they do indeed eat and drink.) And the idea of the act bringing some of these creatures into existence ranges from hilarious (half-giant) to pure Squick. Sometimes, a Wizard really did it, and how!

Web Original

 * Averted in Nemo Ramjet's Snaiad. Snaiad life has right handed amino acids in its proteins, compared to left handed ones in humans. This makes Snaiad life almost indigestible to humans (with the exception of the energy one could get from breaking down sugars and what not). This also goes for the native Snaiadi wildlife, who quickly found out that humans weren't tasty.

Video Games
"Deirdre: Juicy ripe grenade fruits may look appealing, but a mouthful of highly toxic organonitrates will certainly change your mind in a hurry."
 * Justified in Sid Meiers Alpha Centauri, where you have to research the appropriate technologies in order to harvest resources from xenofungus squares. This quote from an in-game character sums it up:


 * Furthermore, although it's mostly All There in the Manual, it's explicitly stated that although terrestrial plants take to Chiron like flies to honey (or perhaps more precisely vinegar) humans must wear oxygen masks and filter their water with ozone lest they die of nitrogen narcosis.
 * Averted in Mass Effect; turian, quarian and volus biology is such that they cannot eat the same food as most other species. Turians and quarians both require dextro-amino acids in their proteins and can't derive nutrients from levo-foods, and at best the food will pass right through, providing no nutrients, as opposed to humans, salarians and asari, who all use levo-amino acids. Volus, meanwhile, have an ammonia-based biochemistry, which is utterly incompatible with everyone else's.
 * A human in the Expanded Universe got sick off alien food designed to resemble earth origin food. Whether this is due to biochemical barriers, or a lack of competence on the aliens' part is unclear.
 * Subverted on Noveria. Most human foodstuffs will make turians deathly ill, but they can eat doughnuts with no adverse effect. Presumably because doughnuts have no nutritional value to any species. See also Rule of Funny; it's a turian rent-a-cop who makes the comment about craving them...
 * In the sequel, there's a romance path for either a turian or a quarian, depending on PC gender.
 * Also in said sequel, one mission involves a group of Alliance soldiers that crashed on a planet and started eating the local plant and animal life. The food caused gradually increasing neurological damage in those who consumed it.
 * Likewise, Grunt claims that the krogan equivalent of alcoholic beverages goes through humans "like broken glass". Shepard can survive having a Krogan drink in one of the hub-world bars, but then (s)he isn't 100% Human anymore. It's also fairly understandable why this is the case; the Krogans have evolved to the point that only a few very strong chemicals can likely affect their biochemistry, and dextro-proteins can still wreak havoc on their systems -- the asari bartender of Eternity on Illium casually mentions that, about 600 to 700 years ago, she saw a krogan drink liquefied turian (that is, a turian that had been blended into liquid) for a bet. In her words, "nobody came out of that one looking pretty". She also warns against the red nuts, as they are for turians and quarians, and will only give humans cramps.
 * You can also overhear a conversation between a Turian salesman and Human customer at a restaurant, in which the Turian explains (And not for the first time, judging by the tone of voice), that you cannot mix spices with the wrong chirality in human cooking, because it will kill him if he eats it. Cut content here shows the Turians don't make much in the way of booze and they are amazed at what wonderful alcoholic drinks they make. Most of the stuff Turians drink for booze stem from Quarians.
 * Used in Sword of the Stars: Humans, tarka and hivers can all eat each others' foodstuffs with no ill effects (though non-hivers find hiver food terribly bland). Hiver biochemistry and taste organs are wired such that human spices and fermented goods (such as cheese) are near-narcotic to them (on the other hand, they have on reaction whatsoever to ethanol), and hiver warriors apparently eat garlic as a form of manhood test because the taste sensation makes it actually dangerous to them. Zuul are able to eat pretty much everything (or anyone) without any bad side effects, though being genetically modified this may be justified.
 * Played straight in Borderlands. The occasional spit-roasted Skag seen at raider camps can be Handwaved as the ill-educated locales (i.e. Rednecks) not knowing any better, but then you find that there is industry devoted to producing canned Skag meat which is quite popular.
 * Played straight with the Mothership Zeta DLC of Fallout 3. Throughout the exploration of the titular alien spaceship the Lone Wanderer finds numerous alien delicacies like squids and worms that when consumed restore your health with no obvious side effects, not to mention there is an alien equivalent to Stimpacks that heals you just as well. Though the game in general does function on what people of the 1950s thought a world after nuclear war would be like, so it is most likely also showing what people from that era thought aliens would be like.
 * In Halo, every alien species thus far (except for the Huragok and Lekgolo, who eat sludge and metal, respectively) are capable of eating humans with no ill effects. Brutes, especially, consider humans very nice eating (at a Nightmare Fuel extent), and grunts, jackals, and drones will happily chow down on PO Ws and KI As. The first human to visit Sanghelios (the Elite homeworld) is served Sangheili food with no problem, although the, um, texture isn't very appetizing ("Who ate that before you?").
 * The Forerunners can safely eat earth-based food as well

Web Comics

 * The huge and scary People-Eating Poly-Sorbate Insectoid in The Inexplicable Adventures of Bob is perfectly capable of swallowing a man-sized creature whole, but cannot digest Earth life. Hence, a swallowed character can punch the critter's stomach lining until he is... forcefully regurgitated.
 * In Freefall, the incompatability of amino acids on Pfouts with terran biochemistry was the reason for Dr. Bowman's experiments with uplifting nonsapient species, in lieu of very expensive space travel and Terraforming. The Bowman's Wolf (including Florence) was to be a "proof of concept" test.
 * Played with in regards to Sam's biochemestry; since he comes from a planet with a relatively benign climate that forced few selection-pressures on his species and lacked anything like the asteroid strike believed to kill off dinosaurs on Earth, his protein structure is surprisingly simple, and Deeee-licious to most Terran animals.

Western Animation
""GIR, your waffles have sickened me! Fetch me the bucket! "
 * Averted in Ben 10 of all things. Ben feeds an alien chocolate, and nearly kills him in the process. This particular alien was a pastiche of Superman, thus making chocolate his kryptonite and only ate it because Ben offered, and he didn't want to seem rude (though neither of them seemed to KNOW that this would happen).
 * Completely averted in Invader Zim. Water is caustic to the eponymous alien, and meat actually fuses to his flesh. He does attempt to build up a tolerance, but GIR, being a robot, can (and does) eat anything.
 * There was one Earth food Zim discovered he could eat (to his own surprise) in the episode Zim Eats Waffles, except the ones made by GIR because he put peanuts and soap in them.

Real Life

 * Caffeine. To us, a mild stimulant. To a bug, a deadly poison of doom. Same for nicotine.
 * Even among mammals, you run into problems. Chocolate chip cookies with macadamia nuts and raisins are a rather easy thing for humans to eat, but to a dog the chocolate will cause potentially fatal arrhythmia and raisins are essentially kidney failure pills, while the macadamia nuts are a powerful paralytic. Some humans don't like onions and/or garlic, but most canines and felines will have their red blood cells dissolve if they eat enough.
 * Nor is it only a problem from one species to another. Cow milk is usually tasty to individuals of Western European stock, but Chinese or Thai groups have an impressive predisposition to lactose intolerance, since after infancy it is the default. It seems that only people from cultures where people traditionally drink milk from cows or other animals are lactose tolerant, since it apparently evolved when we started domesticating animals and taking their milk for ourselves, as being able to properly process the stuff even in adulthood would then have been beneficial. Note that since fermenting milk into cheese or yogurt destroys most of the lactose, people who are lactose intolerant can generally digest those foods much better than they can raw milk. This is why anybody bothered milking farm animals in the first place.
 * Some anthropologists believe that this is why the native Americans turned against Leif Ericson's expedition to North America in 1066. The natives gave the explorers an offering of food, to which Ericson's group responded with an offering of their own -- fresh milk from the goats they had brought with them. Unfortunately, the lactose-intolerant natives grew ill from the milk, and decided that Ericson had tried to poison them!
 * Similar patterns apply for alcohol. To ensure that water was safe for human consumption, early civilizations fell into one of two camps: those who boiled their water (primarily Asia, where dried leaves were added for flavour and was the origin of tea drinking) and those who fermented either juice or sweetened water, creating wines, beers, and mead. (It's not strictly the alcohol: the fermentation itself kills off pathogens, as the quickly dividing yeast absorbs all the food and starves them.) To this day people of European descent have a generally higher alcohol tolerance than Asians do, and even among Europeans cultures that discouraged alcohol drinking will have less tolerance.
 * Also, brewing beer involves using boiling water to leach the sugars necessary for fermentation from the malt. In the same way for making mead, honey dissolves a LOT faster in boiling water, and if any pathogenic lifeform DOES contaminate the fermentation process, it is usually VERY easy to detect it in the product, adding another layer of protection.
 * Pure nicotine applied to the skin will kill any mammal, humans included, in seconds. And caffeine becomes lethal past a certain level of blood content. It's not a matter of biochemical barriers; it's a matter of scale. A study by NASA of all people shows the results of caffeine, marijuana, Benzedrine (speed), and choloral hydrates (sleeping pills) on a spiders ability to make a web.
 * Hilariously spoofed here.
 * This works the other way as well. Death's cap mushrooms are, as the name suggests, potentially lethal (hello, liver transplant!) to humans, but are commonly eaten by other animals, especially deer. In fact, more than one person has died from eating death's cap after assuming it was safe because he saw some other animal eating it.
 * Peppers are hot because they are filled with a chemical called capsaicin (or methyl vanillyl nonenamide, if you really must sound impressive). This chemical is toxic to most small mammals, but not to birds, which also cannot taste it and to some extent acts as an anesthetic. The seeds, likewise, can pass through bird digestive systems, but are rendered non-viable by mammals. The capsaicin, thus, serves by its burning sensation to prevent mammals from eating it, leaving the flesh for the birds which help spread the seeds. Humans are large enough that it doesn't normally affect us this way, but most animals don't actively seek out food that burns their mouths.
 * On a side note, dimethyl anthranilate, which is used as the principal artificial flavoring in grape soda, has the same biochemical effect on birds that capsaicin has on humans.
 * Some avocados are toxic to some birds, and most other animals native to the regions in which they grow. It is hypothesized that the fruits evolved to be eaten by giant ground sloths which are now extinct, which would spread the seeds in their droppings. Nowadays guacamole-hungry humans and their knowledge of agriculture serve the same purpose.
 * There's at least one genus of plant that stores up its energy as a carbohydrate that exists nowhere else in nature, rather than as starch. So far as is known, nothing can utilize this storage-molecule except the plant itself, so no herbivore bothers to try to eat it.
 * Played straight in the story of the Ghost Dance, wherein people dreamed of visiting the moon and eating animals from the moon, making this trope Older Than Radio.
 * There are very very few life forms on Earth that are able to digest plastics. This is why our garbage is expected to last for millions of years before it decomposes. Recent observations of the North Pacific Gyre suggest that we're providing enough food to allow the few we do have to start to thrive.

Anime and Manga

 * In Dragonball Z, Goku apparently contracts a disease while on the planet Yardrat, weakening his heart and which would have eventually killed him. He's saved by a vaccine brought over from an alternate future where he did die. Of course, while Goku is an alien himself, the Saiyan biochemistry is identical to a human being's as far as nutrition and reproduction are concerned.
 * It is speculated that the parasite behind the Hinamizawa Syndrome actually came from another planet. Then again.
 * Crossbone Gundam: Ghost has an alien microbe that the villains plan on releasing on Earth, which the heros are trying to prevent. The logic is that since the microbe has a completely alien biochemistry, nothing on Earth will be able to defend against it, and it will spread like wildfire and wipe out any life it comes across. This totally ignores the fact that the opposite should be equally as true: the microbe should be just as defenseless against Earth's microorganisms. Or, in the most likely case, absolutely nothing happens, and the alien microbe dies due to lack of its native environment.

Comic Books

 * The Green Lantern Arc Sinestro Corps War had a virus called Despotellis capable of killing more than one species, even ones normally resistant to disease. Probably a Justified Trope since Despotellis is sentient.
 * Played reasonably straight in Alan Moore's Top Ten with Sudden Traumatic Organic Rapid Mutation Syndrome or S.T.O.R.M.S. This unpleasent sexually transmitted disease mutates sufferers into a new, invariably non-viable lifefrom ( and in one case a god). Interestingly it is said to be the result of a city full of humans, super human, mutants, aliens, Robots, magical creatures and all the variations from alternate universes all interacting sexually with each other.

Film

 * The Facehuggers in the Alien films are somehow able to impregnate species (like humans and dogs) that would not have been encountered in the environment in which they evolved or by the species that engineered them (the backstory is unclear on which is the case) as long as it is warm-blooded and alive. This is unsatisfactorily explained by claiming that it is capable of adapting itself to the DNA of the host and incorporate features of said host into the Chestburster it produces, but this does not account for how it is able to do so with a completely unknown species with a vastly different biology. If we suppose it was engineered rather than evolved, it seems likely that its creators would have designed it to target a specific species, because making it so adaptable that it could infest completely unknown species would surely mean that it could adapt to infest the species that created them. (Which, according to some sources, is exactly what happened...) As far as in-movie canon goes, Resurrection makes it clear that the Alien and host's DNA are somehow mingled, even in the host's own blood. Of course, that's still crazy from a biochemistry point-of-view (why would aliens even have DNA?), but at least they try to explain it.
 * Somewhat justified by a suggestion that the Xenomorphs were created as either a bioweapon or a terraforming construct (which is basically the same thing, when you come down to it). Step 1 of any biosphere replacement is going to be getting rid of the existing biosphere, and my aren't these efficient at slaughtering things... This idea presupposes that a simple, but non-obvious, kill switch exists in the Xenomorph biology allowing some easy-to-handle chemical to kill them all. Now imagine a strain that mutates so that it doesn't WORK any more...

Literature
"Jos: "Giving a Devaronian two cc's of plethyl nitrate will cure a lobar pneumonia and open up his congested lungs with virtually no side effects. Give that same dose to a human and it'll drop his blood pressure into the syncope zone. Give it to a Bothan -"
 * In HG Wells' original War of the Worlds the Martians are killed off by a terrestrial microbe within several weeks of their arrival. This is almost a reversal of the trope, as it's rather explicitly stated that the aliens are more vulnerable to Earth microbes than humans, due to not having evolved and adapted alongside them; this is based upon the outbreaks that can occur when two previously isolated human cultures begin interacting and exchanging diseases, unintentionally or otherwise. Of course, this is because Science Marches On; the very idea of viral transmittance was new when the book was first written.
 * War of the Worlds manages to hit the trifecta on this. In addition to the above virus example, the Martians are also capable of breathing Earth's atmosphere (to say nothing of the pressure. The only effect is that the higher concentration of oxygen invigorates them!) and "feed" (having "given up their digestive systems") by injecting themselves with human blood. Oh how science has marched on.
 * Possibly averted in explaining why the Martians did not do to us as was done unto them: "Either there are no bacteria on Mars, or else Martian science eliminated them long ago." Wells allowed for the possibility that the Martians had created - and by extension adapted to - a totally germ-free environment and thus left themselves vulnerable to essentially everything. Earthly bacteria are amazingly resilient and resourceful under evolutionary pressure; it's hard not to imagine them finding something about a Martian they could get their flagella into.
 * Thoroughly averted in James White's Sector General book series about an interstellar hospital. (This was someone who would Hand Wave most scifi technologies, but think long and hard about how to perform CPR on a six-foot-long, silver-furred, sentient caterpillar. ) A basic principle of the entire station is that a doctor cannot be infected by exposure to patients of another species. The staff nevertheless remains jumpy at the thought of what a cross-species pathogen could cause if one was some day found. In one book, The Galatic Gourmet the possibility that a cross-species pathogen has not only turned up but gotten loose in the hospital is a large part of the plot.
 * In Robert Zubrin's First Landing, lots of people on Earth start worrying about this after the protagonists announce their discovery of microscopic Martian life. One of the astronauts radios a response back, giving the exact reason why it wouldn't happen -- not coincidentally, she almost word-for-word quotes Zubrin's earlier nonfiction The Case for Mars. (The novel was mainly meant to promote the ideas in TCfM.)
 * In The Krytos Trap, someone engineers a biological weapon that affects many sentient species other than humans. At least partially justified -- it's shown as a difficult task, requiring time and ingenuity, and he mostly targets species groups that apparently share a common origin. The disease is quite horrific, but the same adaptability that let it spread to other species also meant that it quickly became less lethal.
 * Later in the Star Wars Expanded Universe, an engineered bioweapon was developed for use against the Yuuzhan Vong, who were extra-galactic -- there were significant amounts of DNA they had that no known organism in the galaxy far, far away did.
 * Medically, bacta seems to work on just about anything, but other medicines and treatments vary from species to species, as illustrated in the Med Star Duology, a Star Wars novel that serves as a medical drama.

Bariss: "And he'll be dead before he hits the floor.""


 * Bota, called "an adaptogen that can cure anything but a rainy day" does something different for every species, anything from medicine to nutrients to incredibly potent drugs, and briefly raises Force-Sensitives into something very like godhood. Everything on that planet is said to have similar mutagenic power, but mostly what it does is make people sick. By the end of the duology, bota has mutated itself into uselessness.
 * Avoided in Larry Niven's Known Space short story "Madness Has its Place". When the protagonists begin preparing for combating the incoming Kzinti, biological, chemical, and nuclear weapons are suggested. The main character promptly strikes down the first one; there was no guarantee that a bio-weapon that worked against humans would work against eight-foot tall war cats (if anything, they should capture some of their biologicals).
 * Also averted in the Ring World books, in which inter-species sex (called rishathra) is common, and it is explained that STDs aren't an issue. Disease transmission would be a little more plausible in this case than in most of the other examples, because.
 * Apparently nobody had told Niven that new diseases could evolve from common soil bacteria, when he first introduced rishathra to the Ringworld's cultures. Until The Ringworld Throne, he'd claimed the  simply didn't put any pathogens there, V.D. included.
 * In Arthur C Clarke's Before Eden, astronauts discover lithovore slime-like life on Venus... only to wipe it out by inadvertently introducing pathogenic microorganisms from Earth.
 * The Vorkosigan Saga again: the notorious and disgusting worm plague of Sergyar. "It wasn't all that lethal, as plagues go."
 * In the Ender's Game Speaker for the Dead trilogy the humans and their crops are infected by a virus, the Descolada, on the alien planet. This trope is averted by the facts that.
 * In the Honor Harrington series, this is part of the backstory for Manticore -- a few years after the colonists arrived, one of the local microbes crossed the species barrier and killed a substantial portion of the colonists. Somewhat justified in that one of the things that made Manticore inviting to colonize was how similar its biochemistry was to Earth's. Also, the original colonists' desire to maintain control while bringing in replacements for everybody who died is how Manticore ended up with a feudal system of government.
 * In the same universe, the capital planet of the Anderman Empire had a native microbe that was harmless to humans but ate chlorophyll. The colony was slowly starving because of crop failures, when super-rich mercenary commander Gustav Anderman came along and paid for the expensive genetic engineering to make resistant plants, in exchange for being made emperor.
 * Weber averts this again with the planet Grayson, a Death World full of heavy metals which forces them to do all their agriculture either under protective domes or on orbital farms.
 * Hades/Hell is effective as a prison planet exactly because its wildlife couldn't be eaten by humans and the vegetation is eventually fatal, so inmates are completely dependent upon their wardens. Camps that misbehave have their food supplies cut off. Also humans from different planets have different tolerances. The heroine has to be careful about Grayson foods since even the ones grown in protective domes can have more heavy metals than she can handle while at least one Grayson is allergic to squash from her homeworld.
 * The Star Trek Expanded Universe includes a series dealing with a plot to infect all Alpha Quadrant species with 100%-mortality diseases. The first book in the series presents a virus that specifically infects characters of mixed heritage.
 * Not exactly a disease, but in Silent Dances the main character avoids getting bug bites on the alien planet because the bugs can't handle her alien chemistry. On the other hand, she has no trouble eating most of the plants and animals on that planet for food...
 * Averted in the Corean Chronicles for good reason. Virtually all of the native fauna and flora were less animals and plants than rocks and gas interwoven with energy; the necessary jump from silicon- to carbon-based lifeforms or vice versa would be a major barrier.
 * In one Animorphs book, Ax catches a disease. All of the human Animorphs but Cassie come down with apparently the same disease, which manifests as the flu in humans (though it's much more potentially fatal to Ax.) Worse, the disease seems to affect everyone even in morph, including Tobias, who spends very little time in human form.
 * In David Starr, Space Ranger, a series of poisonings occurs in people who ate Mars grown food. A (human) Martian scientist says it could have been a poisoning by the local bacteria.

Live Action TV

 * Babylon 5 had the Markab people fall victim to a universally fatal plague to which humans turned out to be completely immune, as it acted on a type of nerve cell that humans don't have. The Pak'ma'ra, however, turned out to be similar enough that it also affected them, though less severely. In fact, that similarity allowed Dr Franklin to figure out how to cure the disease -.
 * In an episode of Stargate SG-1, the local alien population of a planet are falling ill after the arrival of the main team. As the good guys desperately try to find a cure, General Hammond points out that they're lucky they mostly deal with human civilizations who have the same diseases as Earth do and that's it's a small miracle they haven't run into a problem like this before. The episode was ultimately an aversion, since it turned out that the illness wasn't caused by a disease.
 * The Goa'uld are a race of parasitic worms that evolved on some distant planet, but seem to be capable of infecting every sentient race they come into contact with, without any discernible difficulty. They supposedly need to acquire the genetic code from a species they're going to infest and apparently do this via sex, which is...odd, at least the first time. Also, the Jaffa were initially created to allow larval Goa'uld time to adjust to human hosts. (Prior to the Jaffa, many more Goa'uld died of rejection sickness.) The Goa'uld still can't parasitize some species, such as the Retou. It's also explicitly noted a couple times that some species or human populations are resistant or immune to Goa'uld infestation, but the Goa'uld make a habit of wiping them out.
 * A Doctor Who episode had a human stung by the venomous tongue of a Silurian warrior, and begin to mutate. This trope is simultaneously played straight (it does affect him), subverted (the Silurian doesn't understand why he doesn't just die) and partially justified (Silurians and humans are both technically earthlings; they are just separated by millions of years of evolution).
 * The Girl Who Waited plays with this; the 'One Day Plague' only affects species with two hearts, so Time Lords and Apalapucians are at risk but humans are fine. Also, when Amy is trapped in a quarantine facility, the Doctor instructs her not to accept any medicine from the robotic staff; they can't comprehend that she's a different species to the rest of the inhabitants and any medicine they give her would be lethal.
 * Star Trek Enterprise had an episode where a disease that was apparently a universal infector was used as a sociology experiment by an alien race that had surpassed physical existence -- they wanted to see what cultures would do if infected by an incurable airborne alien virus that killed quickly.
 * The Star Trek novel Uhura's Song was all about finding the cure for an epidemic striking both humans and the catlike Eeiauoans, complicated (among other factors) by the fact that nobody on the planet that should hold the solution recognized its symptoms.
 * The Star Trek the Original Series novel The IDIC Epidemic concerned a highly virulent and rapidly mutating disease that infected everyone living on a mixed-species treaty world. Originally a Klingon disease, it would affect everyone with similar blood chemistry (iron-based, copper-based, silicon-based), and leap from one blood chemistry to another via mixed-species children. In the end, Romulans (copper-based blood) were immune, and the Klingons (iron-based blood) had an ample supply of the cure, and mass inoculations saved the day.
 * Averted in an episode of Battlestar Galactica. The fleet stumbles across a Cylon basestar where all the Cylons on board are either dead or dying. It turns out that they've contracted a disease that humans became immune to millennia before due to.
 * Unfortunately played straight when
 * Mighty Morphin Power Rangers: In the opening three-parter of the third season, the Repellator is briefly afflicted with Kimberly's cold after she sneezes on him a couple times. Lord Zedd's reaction is hilarious.

Tabletop Games

 * The Life-Eater virus in Warhammer 40000 fluff can scour entire planets clear of life in minutes, no matter what kind of life inhabits said planet. Even Eldar (with quintuple helix DNA) and Tyranids (which may have local DNA salvaged from corpses, but the race as a whole comes from a different galaxy).
 * The fluff implies that it's a sort of nano weapon.
 * The Tyranids are a special case. Given what they are - a horrendously adaptive Horde of Alien Locusts implied to have devoured entire galaxies - they will go through several evolutionary cycles in a matter of weeks to adapt to a local ecology.
 * Nurgals Rot is another fluff based disease that can effect all life, justified because the disease is also created by the god of plague. it does not kill you but rots your body and soul in order to drive you to despair.
 * Played with in the Classic D&D game's diaboli, a race from an alternate dimension where many aspects of the cosmos are reversed. Not only are the normally-harmless bodily secretions of diaboli deadly poisonous to humans, but human saliva is equally toxic to them.

Video Games
"Mordin: "Implications... unpleasant.""
 * In War Craft III and World of Warcraft, although the Plague of Undeath created by the Lich King only affects humans and not any of the other intelligent species in the game universe, it still has mutagenic effects on plant and animal life. Probably because it's magic and was made to prepare the land for undead invasion. In at least one quest it's explicitly stated to affect some other races, but not fully as intended; one quest involves getting bones from Murlocs that were turned into undead by the plague... but weren't robbed of their free will by it as humans would be.
 * Warcraft also has Half Human Hybrids, despite diseases apparently not translating well among species on the same planet. For example, Garona Halforcen who is half orc and half DRAENEI. Yup, alien parents from different planets. And then she had a son with MEDIVH, who is, biologically, pretty much human. So that's 3 species from 3 different planets who are all compatible with one another. Draenei and Orcs in particular are interesting, since their hybrids are clearly still fertile.
 * When the Lich King first came to Northrend, he had difficulty fighting the Nerubians because they couldn't be infected and killed directly by the plague, but they weren't immune to the Scourge's Necromancy, so once some of them got killed by non disease means, they were able to be resurrected as undead slaves.
 * Played with in Mass Effect. The quarians have impaired immune systems and have to wear suits so they don't get sick. At first appearance, that's this trope, however, Tali explains in the sequel that it's not really an infection: it's an allergic reaction as their body tries to adapt to the foreign substance in their system, the quarians having evolved on a world where all microbial lifeforms were at least partially beneficial and so their immune systems evolved to assimilate the virus, not destroy them like in other species. To use chicken pox as an example, if she were exposed to it, she wouldn't catch chicken pox. She would have an allergic reaction, with similar, flu-like symptoms. They can also take antibiotics and temporary immune boosters to fight infections if their suit gets ruptured (or if they want to take it off to have sex).
 * In addition to the nature of their home planet's fuana, Quarians have spent the past few hundred years in the completely sterile environments of the Flotilla's ships, and so their immune systems have only continued to grow weaker. In the third game.
 * In the first game, the ship's VI makes a point of decontaminating the crew every time they come aboard. This is actually justified, since there are humans at most of the planets that Shepard visits, and there is the possibility (however small) of a cross species disease.
 * Used as a plot point in Mordin's recruitment mission: everyone you speak to in the plague zone knows the disease ravaging the area has to be an intentionally released bioweapon precisely because it's infecting every species except humans (and vorcha, but they're immune to everything).
 * Also used humorously in a throwaway line by Mordin on the Normandy, where he mentions he's trying to figure out how a "Scale Itch" infection got on-board... considering it's an STD carried only by varren.


 * In Sword of the Stars, the disease bio-weapons need to be invented only once and are equally useful against all species. They can be fired in a first-contact situation (before you'd logically be able to dissect a member of said species and find their disease markers) and will not lose any efficiency. The only exception is the Zuul, who are immune to all plague weapons except for the Grey Goo missile no matter what you try.

Western Animation

 * Justice League had a rogue amazon trying to wipe out all males on Earth with an engineered gender-specific virus. It even affected Superman and the Martian Manhunter, who aren't human (The latter barely even qualifies as male by human definitions). The worst part is that the "disease" is finally stated to be an engineered allergy. Allergies do not work that way.
 * Somewhat averted in Invader Zim, where when lice broke out, Zim was the only one not infected (Well, there was Miss Bitters, but she ended up getting them at the very end, too.) Turns out there's something in his skin that kills the lice. This is probably the only time in the show where something earthly wasn't harmfull to him in some way. Of course, it might be possible that Irk once had lice or a similar creature, and their lice killing skin is an adaptation.
 * Also somewhat played with in "Germs," where Zim sees a movie about Earth germs killing alien invaders and becomes insanely mysophobic. The germs never do seem to hurt him, though, implying they probably can't.
 * Star Wars Clone Wars has a virus called "the blue shadow" which is highly contagious to all species. This ability is exactly why it's so feared in universe, as not only is no one immune, it's extremely deadly.

Web Comics

 * Not disease, but medicine - Schlock Mercenary averts this nicely by having characters have med-kits in their tanks specific to their species. These so far have not worked on other species, though there haven't (yet) been any incidences of adverse effects. Medical nanites are more-or-less universal, however.

Real Life

 * Several diseases can do this. The most notorious include rabies, mad cow disease, foot-and-mouth disease, and the infamous avian flu. There's also distinct possibility that virtually all notable human diseases originated from livestock vectors. Smallpox and anthrax are of bovine origin, influenza of avian and porcine stock, the common cold may be from horses, HIV a variant of SIV from African green monkeys, etc. The only ones that don't show strong relationships with livestock are STD's, with the exceptions of HIV (as above) and syphilis (originally limited to sheep). But on the whole, these cross-species diseases are still fairly limited. Rabies, for instance, is stunning in its ability to cross species lines, but is still limited to infecting mammals. It would be unlikely to be found in a reptile, inconceivable in an octopus, and beyond ridiculous in an alien.
 * Mad Cow, more correctly known as Bovine Spongiform Encephalopathy, is a special case as it's not caused by even a rudimentary lifeform such as a virus. BSE is a prion-based disease. Prions are mis-folded proteins that are nonetheless stable enough to interact with a biological system, and several are known to causes diseases. Since proteins are one of the most fundamental building blocks of earth life, it wouldn't be surprising to find prion-based diseases capable of infecting a wide range of earth lifeforms. BSE itself is known to have variants that can infect cows, horses, sheep and humans.
 * Even stranger are Bunyaviruses, Reoviruses and Rhabdoviruses, which can infect both animals and plants.
 * As reasonable as the assumption that Space Germs Are Incompatible With Terrestrial Life Forms may be, it remains a hypothesis until we actually find a Space Germ to use in experimental verification. Since, ideally, one doesn't want to use the entire Terrestrial biosphere as the lab for such an experiment, NASA has a long tradition of quarantine periods for returning astronauts. They also do their best to thoroughly sterilize any outgoing space probes, to avoid contaminating fragile extraterrestrial biospheres. A new host may not have adequate immune defenses against a new infection or infestation, but the parasite/pathogen won't usually be pre-adapted to attack the new host, either. Sometimes the invader won't find anything useful to "eat", or will be defeated by environmental factors such as higher body temperature, but if it survives it may just as easily be able to pig out on undefended tissues. Of course, at this point we still have only Earth organisms to base studies on. Parasitic and bacterial infections are more likely than viral ones, as the former are (in a sense) basically "eating" parts of the host. Viruses "eat" cells only in a far less literal sense, requiring a certain degree of DNA compatibility to replicate.
 * While bacteria might theoretically be able to infect an alien host, a virus would probably not. A virus needs to recognize a particular DNA sequence to graft itself into the right place on the host's genome, so that it can trick the cell's reproductive machinery into making copies of itself. Even if the alien has DNA -- which itself is questionable -- there is very little chance that any sequence in its DNA would be of reasonable length to match the pattern that the virus needs, and even if the virus was able to adapt it would still need very specific circumstances that are likely unique to Earth.
 * Going back to the mention of space germs, the notable astronomer Fred Hoyle believed that human noses evolved to point down in order to help keep us from inhaling any microbes that fell from space. Mind you, he was rightfully laughed at for that hypothesis.

Anime

 * The planet Namek in Dragonball Z has a green sky. That alone should point out the differences in composition with Earth's atmosphere, but the humans that visit this planet, and the Namek natives that visit Earth (and the multitude of other aliens from many different worlds that congregate on either planet) have no problem breathing in it or even fighting at full strength.
 * There's also a scene that lampshades this, where Bulma begins analyzing the atmosphere from inside their ship to see if it's breathable - only to look up and see that Gohan and Krillin are already outside.

Comic Books

 * Inverted in The Scrameustache. Human Aliens wear protective gears on earth because of our pollution. But those who spend too much time on Earth adapt.
 * A Fantastic Four comic has The Thing and Mr Fantastic visit a alien space station with an acidic atmosphere. After having his helmet broken it turns out The Thing can breath the air, it just tastes awful.

Film
"Guy: What're you doing! We're on an alien planet! Is there AIR? You don't know!
 * Same in Star Wars, where a bunch of different species mingle with humans without respiratory problems. For that matter, it seems likely that various human populations would adapt to the particular atmospheric conditions of their homeworld over the millenia and that they therefore could find the conditions on other worlds tolerable but bothersome. There are some exceptions, though, as a few aliens are shown wearing breathing rigs, and the Expanded Universe adds more--including at least one which doesn't even technically breathe.
 * It is mentioned that the human Rebels find the Mon Calamari battleships annoyingly warm and humid (Mon Calamari are amphibious and will dry out quickly), but nothing worse than that.
 * Gravity gets a similar minor mention, in that humans who grew up on high-gravity worlds tended to be stocky. Otherwise it really doesn't come up.
 * The EU from the novels goes a bit more in depth into this with the Gand (Technically two species). Group one of the Gand must breathe methane or they will enter a coma state. Group two no longer needs to breathe. Then there's the case of the Givin, a race that evolved on a planet with a horrific environment, including massive gravity shifts caused by a destabilized moon. The Givin have an exoskeleton that allows them to survive in vacuum for short periods (Their planet would have its atmosphere shredded on a regular basis) and an amazing capacity for mathematics that helped them to predict when these gravity shifts would occur.
 * Most of the aliens in the Men in Black films and the cartoon get along fine without special respirators, although there are exceptions: one species risked premature aging in earth's atmosphere, while "Atlantis" (actually a sunken space ship) is a posh resort for (salt)water-breathing aliens. When Agent Jay is surprised at the fish-like aliens, Agent Kay lampshades this with "You think all aliens breathe oxygen?"
 * Lampshaded and played hilariously straight in Galaxy Quest, when Tech Sargent Chen opens the door of the ship while it's on an alien planet:

Chen: (sniffing a few times) Seems okay."


 * Present and subverted (and generally just plain screwed up) in M. Night Shyamalan's Signs. Aliens are seen walking around on Earth apparently sans any kind of protection whatsoever, but then turn out to be fatally allergic to water. (But let's not go into why hydrophobic aliens would decide to invade a planet 75% covered with water, nor how they managed to run through a misty cornfield at night.)
 * Partly averted in Avatar. Although the atmosphere is primarily oxygen/nitrogen (albeit denser), humans can not breathe it without filters due to the presence of lethal levels of carbon dioxide and hydrogen sulphide.

Literature

 * Averted in the Robotech II: The Sentinels novels. The Garudan's homeworld is oxygen-rich much like that of the Humans and other aliens, but also contains chemicals and microbes that for the other races are basically the equivalent of breathing LSD with your air.
 * Sector General has oxygen-breathers, water-breathers, chlorine-breathers, methane-breathers, cryonic methane-breathers, oxygen/micro-organism breathers/eaters/drinkers and whatever-the-hell-the-radivores-breathe-breathers, and the Hudlars (classification FROB) require a different kind of oxygen-atmosphere, having evolved on a high-gravity planet with a very thick, high-pressure oxygen atmosphere (the make-up of which is an occasional minor plot point, albeit purely because Hudlars "eat" by absorbing micro-organisms and nutrients through their skin). However, the extremely tough tegument they are covered in (among other things) means that they can actually survive in a vacuum if need be, provided they get sprayed with nutrients often enough.
 * Averted in Timothy Zahn's Blackcollar. The hero has an allergic reaction to the atmosphere of one planet, requiring him to swallow medication in regular intervals or suffer an asthma attack. Of course, in the same novel the evil aliens have no problem breathing the same atmosphere as humans.
 * Averted in Algebraist, which avoids all of these categories, but especially this one. If you want to survive in someone else's environment (whether their planet or a space station) you better bring a suit. Suits are not always mentioned, because it's so ubiquitous, it would be akin to mentioning that a person is wearing clothes. Alien environment, alien suit. This of course means alien breeding, cuisine and diseases are right out.
 * Slightly averted, yet mostly played straight, in Julian May's Galactic Milieu Trilogy/Pliocene Exile saga. The Ship specifically searched for a highly compatible world for the Tanu/Firvulag, only finding one several galaxies over and dying in the process of getting them there. Later, the Milieu is largely composed of mostly biologically compatible species - the Krondaku are uncomfortable on Earth, as it lacks enough oxygen and has twice the gravity of their homeworld (although it supplies some of the galaxy's top-notch booze); the Gi are also a light-grav species (although more socially incompatible); normal enough for the mostly photosynthetic Simbiari (although high concentrations of CO2 are an intoxicant); eminently compatible for the Poltroy, if a little warm and lacking in those so-tasty sulfites; it's a good question if the Lylmik even have a metabolism, being mostly incorporeal, but if they ever did it was pretty wild (the few glimpses of their ecosystem are alien in the extreme, and explicitly stated to be artificially constructed to be at least slightly compatible with all the other races); and the strangest human of the lot, Jack the Bodiless, is perfectly compatible due to the fact that he requires little more than light and the odd organic molecule, and is otherwise completely unaffected by the environment.
 * Also, a Krondaku love-bath is hallucinogenic to humans, and would serve well as a medium-grade machine oil, being mostly liquid hydrocarbons.
 * Averted in a Nicholas Fisk short story. Human spacemen discovered a cute photogenic alien on a hostile unpleasant planet. They took it home, carefully replicating the environment they found it in. Turns out the sentient alien was an explorer, only able to cope with the hostile environment for a short time. As the alien transformed from cute and photogenic to sick and ugly, the humans lost interest and it was able to escape. Turns out earth rain and atmosphere were perfect conditions for recovering. Not to mention food for the soul in a friendship with a little kid.
 * Subverted in The Martian Chronicles, when the parent Martian at the beginning of the book assures the child that there can be no life on Earth because the planet has too much oxygen in its atmosphere.
 * Averted in Lois McMaster Bujold's Vorkosigan saga, where people only terraform planets with enough oxygen that they can tweak the atmosphere to the Earth norm. On Komarr, the same atmospheric chemicals are present but oxygen levels are much lower, requiring breathing equipment if you go outside. (There are no intelligent aliens; most alien life-forms we meet are similar to insects.)
 * Averted in the Corean Chronicles. The native climate of the world was hostile to humans, especially in terms of temperature. Modification of that climate drove the native species to gradual extinction.
 * Averted hard in Larry Niven's The Draco Tavern. Either specify Tee Tee Hatch Nex Ool (or Tefee Tee Hatch Nex Ool for some lighter gravity) before you take your suit off, or die horribly.
 * There is a short story by Isaac Asimov where a race of aliens can breathe quite well on Earth... so long as they have a flask of hydrogen cyanide to breathe from every few minutes. Another story has aliens who, when they visit human planets, require filters to absorb carbon dioxide - it is highly toxic for them.

Live Action TV

 * Star Trek is a frequent offender here. Given that the majority of the crews found aboard the ships the various series center around are human, it is reasonable to assume that the air they breathe is adapted to human physiology. Other oxygen-breathing species are almost never shown to find the pressure, temperature or the (presumable) presence of nitrogen and water vapour bothersome. The Benzites are the only race that's been described as needing something other than oxygen/nitrogen. It's possible the Breen count too, since those refrigeration suits aren't needed, as well as the Tholians, who naturally live in temperatures of at least 450 Kelvin (177 Celsius). This is given a Hand Wave by an episode of Star Trek the Next Generation, which explains that most of the species in the Alpha Quadrant did in fact descend from the Precursors, who scattered their genetic code across thousands of worlds, apparently Genre Savvy enough to know that their own society would inevitably grow proud. This goes some way to explaining why Rubber Forehead Aliens tolerate nitrogen/oxygen atmospheres, as well as other forms of alien/human... interaction.
 * There was a nice moment in a Deep Space 9 episode that seemed to acknowledge this, where Garak, a Cardassian, comments that the environmental settings on the eponymous station, adapted to Bajoran/human norms from their Cardassian originals, are uncomfortable to him - "the temperature is always too cold, the lights are always too bright". So even amongst Star Trek's rubber forehead brigade there are still variations in environmental tolerance.
 * A plot point of the TOS episode Amok Time is that Vulcan's atmosphere contains less oxygen than humans are used to. Also, when afflicted with rapid aging in "The Deadly Years", Spock found it more difficult to tolerate the (for Vulcans) cold temperature of the ship environment. Presumably, he normally just puts up with the discomfort.
 * Although fine in Earthlike temperature ranges, Klingons have more difficulty dealing with colder temperatures than many species.
 * Pick any story of Doctor Who out of the TV series, novels and comics. There's a good chance this is happening.
 * For example, there's the Calvisano, Weeping Angels, a Multiform, Alfraxi, as well as a Time Lord and humans themselves all breathing alien atmosphere within the last five episodes.
 * Justified in Stargate SG-1. As Carter points out, you don't make easy travel to planets that are going to kill you if you go there. Also subverted several times when they do run across lethal atmospheres, usually caused by some kind of disaster like a volcano. In one case it was a radically different form of life terraforming the planet. They always send through a robot probe before going to an unknown planet.
 * Also Justified because the Ancients had the same or very similar chemical composition as modern humans, and deliberately built their Stargates on planets they could inhabit in the Milky Way.
 * Averted in Babylon 5 in that there are sections of the station sealed off for species that breathe atmospheres other than the oxy-nitrogen standard that seems to be common among the humanoid races. Some of these zones were mild enough that humans could enter them with just respirators, while others (never seen on screen) were apparently nasty enough to require full pressure suits. This difference is mostly informal, since except for the encounter suite-wearing Gaim and Vorlons and whatever Na'grath is supposed to be, any alien we see on Babylon 5 is effortlessly capable of surviving in earth-like atmospheric conditions.
 * In UFO the aliens have the technology to adapt human organs to replace their own, yet wear spacesuits as exposure to the Earth's atmosphere will kill them.
 * Farscape usually played this trope straight in all its incarnations but the atmosphere issue is subverted in one episode. In "Relativity" the alien mercenaries employed by Xhalax are used to much less air pressure so they have difficulty keeping up with her.
 * Power Rangers as a whole. Not sure whether the ranger's armor acts as a protective gear (surely it does in Power Rangers in Space). But anyway, humans, Human Aliens good guys and Dark Specter's forces can breath on each other's planets. Oh, and apparently you can breath on Moon, and it has the same gravity as Earth.

Tabletop Games

 * Averted in GURPS: Space most of the possible atmospheres are both suffocating and toxic.
 * Averted in Dungeons and Dragons when traveling the planes. Depending on the plane you go to, problems may include but are not limited to: having the life sucked out of you, too much life being crammed into you, obscenely high temperatures, obscenely low temperatures, lack of atmosphere, poisonous atmosphere, or in some extreme cases whatever the DM feels like screwing the players over with.

Video Games

 * Mass Effect deserves a third mention. This time, it's the volus. Apparently, the volus homeworld is a high-pressure cocktail of ammonia and other caustic gasses; all the volus we see in-game wear high-pressure suits, and it's implied that, without them, their skin would split open from their bodies' internal pressure. Ew.
 * There's also a planet you can go to on one of the side missions that is very close to being Earthlike. It even actually has MORE oxygen than Earth...which is part of the problem as the thunderstorms on the planet are magnified to horrifying intensities. This plus toxic pollen rampant in the atmosphere makes it thoroughly unsuitable for human life.
 * In the first game, a good percentage of the planets you land on are hazardous. The hazard can be due to extreme temperatures, extreme pressure, or atmospheric composition that's toxic to your team.
 * Mass Effect 3 makes mention of the fact that Rachni typically exist only on planets that are exceedingly hazardous to most other forms of life, and finding them on a planet with an atmosphere comfortable for humans and Krogan is rather unusual.
 * Averted with the Grunts of Halo. The massive suits and rebreathers they wear provide them with the methane atmosphere they require. All the other species seem to share in being adapted to the same atmospheric conditions as humans, which makes sense since most of the other sentient species were specifically preserved by the Forerunners, who lived on Earth-like planets. It's also implied supplemental material that Grunts may have evolved to breathe methane after they wrecked the atmosphere of their planet with oveindustrialization.
 * Averted in Pikmin. It's stated several times that the atmosphere of the planet Olimar gets stranded on contains a large amount of oxygen, which is highly toxic to his species. This is also why he has to repair his ship in thirty days, as this is how long he has before his suit's life support runs out.
 * Averted in the Sid Meiers Alpha Centauri: Alien Crossfire expansion pack - Progenitor and human biological needs are so different that they cannot coexist in colonies together. In game terms, what this means is that when a human faction captures a Progenitor colony (or vice versa), the colony is depopulated to 1 population, and a number of colony pods (based on the population of the colony) are generated, representing the displaced population.
 * Also there in the manual for the game: the atmosphere of Planet is also nitrogen/oxygen, but the proportion of nitrogen is 90% instead of 78%, and the atmosphere overall is thicker so that while the partial pressure of oxygen is within acceptable limits, the amount of nitrogen you'd have to breathe in would give you nitrogen narcosis. Settlers are mentioned wearing special masks at minimum, and your soldiers are depicted wearing what appear to be pressure helmets. The tie-in novels mentioned that it was possible to go without a mask for short periods of time, so Deirdre made quite an impact on another faction by emerging from her vehicle, unmasked and smiling.
 * Averted in the Metroid series: after the Chozo took the infant Samus Aran under their care, they subjected her to a carefully designed series of transgenic modifications with Chozo DNA, so she could merely survive in the unbelievably hostile atmosphere, and pressure, of Zebes. Even then, many parts of Zebes, to say nothing of other planets, are impassable to her without the Power Suit's environmental protections.
 * Possibly played straight in the Metroid Prime games, as the Federation Troopers also consistently wear powered armor everywhere Samus goes (so their armor presumably provides similar environmental protection and breathable air), but Admiral Dane never wears as much as a facemask --not even on the Pirate Homeworld, which has acidic rain that can destroy the Power Suit in seconds.
 * In Final Fantasy IV, the party goes from the surface of the planet to the deep underground --with oceans of lava bright enough to provide ambient light. Then they visit the surface of the Moon, a barren wasteland populated by human-sized versions of (single-cell) eukaryotes and prokaryotes. A case could be made for the latter, but the former should have a noxious atmosphere and should make it impossible for humans to survive in it (to say nothing of the heat and pressure...)
 * X-COM: UFO Defense (UFO: Enemy Unknown for you Brits) both averted and played this straight. The eponymous aliens comprised many races - some were surgically altered to deal with the atmosphere, others were genetically engineered badasses, some used their Psionic powers to support their atrophied body mass... and some just seem to do fine without any help. Including, apparently, humans on Mars without full environmental armor. Possibly an oversight, since by that point in the game, you should probably have all of your troops outfitted with Powered Armor and plasma weapons.
 * Starcraft 2 plays this straight during the invasion of Charr. Despite the fact that the entire world is a giant volcanic hotspot, the surface consists of compacted ash and solidified lava, and open pits of lava are the equivalent of oceans... humans can still walk around with open helmets. And the rain is apparently perfectly neutral water rather than the expected acid rain. The Zerg might have modified the atmosphere more to their liking, but considering they can do fine in a vacuum and evolved on an even worse world, that seems doubtful.
 * Amusingly, General Warfield had specifically stated earlier that the atmosphere could "burn a man alive".
 * Averted in the Myst Verse, in which one of the major guilds of the D'ni civilization had the responsibility of checking newly-written Ages to determine if they were safe. The first explorers to link to an undocumented or rediscovered Age would do so in protective armor with its own air supply.
 * Sword of the Stars at least averts this one, as all races have their own preferences for atmosphere, gravity, humidity and temperature that makes it difficult for two or more species to share a planet without high-level environmental engineering tech. The preferences are randomized each game, which can have interesting results.

Western Animation
"The air isn't toxic...? How DARE you open a space ranger's helmet on an uncharted planet! My eyeballs could have been SUCKED from their sockets!"
 * Lampshaded for comedic effect in Toy Story. Woody opens Buzz Lightyear's helmet accidentally, and Buzz starts choking, expecting something deadly. He suddenly realizes...


 * Even funnier when one considers that he doesn't even have lungs, being a molded hunk of plastic.
 * Cybertron in Transformers is shown to be equally hospitable to humans, various aliens, and robots with no need to breathe. This is despite there being no visible atmosphere in most of its depictions.
 * It is also the altmode of the Physical God who created the Transformers, and who seems to encourage them cooperating with other species, especially humans. Retroactive Fridge Brilliance perhaps?
 * One of the very first things we see on the planet, in the pilot, is a huge fire. Later episodes revealed that under the surface, there is organic earth to be found, and if we go all the way trough Beast Machines, it is out-right stated that plant life did indeed live on the planet. The lack of visible atmosphere in the artwork seems to be the biggest problem, as everything else suggests there is one.
 * Invader Zim again. Irk, Foodcourtia, Earth, and Conventia don't all need to have the same pressure and those there need not all breathe oxygen as, at least in the case of the Irkens, they have atmospheric processing units (in the PAK). No matter what planets the Irken Invaders go to, they can always handle the atmosperic pressure. Of course, it may be possible that many of them are wearing their invisible space helmets, but that's kind of unlikely. Until, of course, it rains.
 * Fairly Oddparents. Cosmo doesn't quite understand what's wrong with visiting 'The Planet Of Not Enough Atmosphere'.

Real Life

 * Fish, squid, etc. can extract oxygen from water just fine, while humans and any other animal with lungs can't and will drown. On the flipside, dry/collapsed gills cannot function. And they dry very fast. There are species of fish that can survive prolonged exposure to air, and just a slight chemical difference in the water can kill an organism with gills. And never transplant a saltwater fish to a freshwater, or vice versa. Unless it's a salmon, and even then they can't move directly from salt water to fresh water or vice versa; they need time in an area that is a mix of the two.
 * Humans can tolerate all kinds of pressure and gas differences, as long as whatever we're breathing contains an approximate partial pressure of 3.0 psi oxygen. Of note, heliox (helium-oxygen mix, containing no nitrogen at all) has long been popular with technical divers at extreme depths, as the lack of nitrogen removes one possible avenue for pressure toxicities. On the other hand, oxygen toxicity becomes a real problem at those kinds of depths, so you have to carry multiple mixes, in separate bottles with separate breathing rigs. And God Himself can't save you if you breathe from the green bottle (100% oxygen) at deeper than about 20 feet of salt water.
 * Even breathing from liquids isn't impossible if they contain sufficient amounts of oxygen (ala The Abyss), although usually only with mechanical assistance as human lungs don't have the power to move the necessary amount of liquid substance to get enough CO2 out and enough oxygen in, and if you spend too long in the oxygenated fluid your diaphragm will eventually tear from the strain of having to move the much denser (compared to air) liquid in and out of your lungs.
 * For an air pressure example, humans have climbed Mount Everest without oxygen tanks. It is not for everyone and you can get away with being there briefly, but you won't survive if you stay.