No Biochemical Barriers: Difference between revisions

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* '''[[Alien Lunch|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. ([[To Serve Man|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]].

* '''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.

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.

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"[[Never Live It Down|You're never going to let me forget it, are you?]]"<br />

"Not a chance, Green-Eyes." }}

* 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.

* 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 [[Animorphs|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 [[Sense Freak|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.

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* 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, ''[[Posleen War Series|The Legacy of the Aldenata]]'', the Posleen are a ravening horde of [[Extreme Omnivore~~|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 [[Neglectful Precursors|Aldenata]] were trying to modify them into a ''less'' violent species. [[Nice Job Breaking It, Hero|Oops]]...

** Also used and averted in another [[John Ringo]] series, ''[[Posleen War Series|The Legacy of the Aldenata]]'', the Posleen are a ravening horde of [[Extreme Omnivore]]s 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 [[Neglectful Precursors|Aldenata]] were trying to modify them into a ''less'' violent species. [[Nice Job Breaking It, Hero|Oops]]...

** [[Lampshade Hanging|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<ref>[[Science Marches On]] may not be kind to this character's plan. Modern food science is discovering that you can fool the metabolism like that for a short while, but humans eating filling but indigestible food acquires a '''terrible''' taste memory very quickly. This is why a bunch of Fat Substitutes failed in the late '90s</ref>, as you feel full, but your system can't process the food. You excrete everything and keep ~~nothing--rapid~~ weight loss happens then.

* 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,<ref>[[Science Marches On]] may not be kind to this character's plan. Modern food science is discovering that you can fool the metabolism like that for a short while, but humans eating filling but indigestible food acquires a '''terrible''' taste memory very quickly. This is why a bunch of Fat Substitutes failed in the late '90s</ref> 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 ''[[Worldwar]]'' [[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 {{spoiler|the meat or bodily fluids of every animal on the planet are deadly poisonous to humans--but then, it also works the other way around.}}

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=== [[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 [[~~Terraform|terraforming~~]] it without contamination.

* 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 [[terraform]]ing 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?).

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** In the sequel, there's a [[Optional Sexual Encounter|romance]] path for either a turian or a quarian, depending on PC gender. {{spoiler|In the case of the Quarian Tali, she actually contracts a mild allergic reaction to the male PC (but it was worth in in her opinion), while with the Turian Garrus, a female PC is cautioned by the ship's doctor not to... ingest.}}

** 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 [[We Can Rebuild Him|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.

** 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 [[We Can Rebuild Him|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 [http://www.youtube.com/watch?v=mGKQhuDW6B4&feature=related 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, [[Lizard Folk|tarka]] and [[Bee People|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.

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** 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!

*** 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.

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** ''[[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. <ref>''With gusto.'' Without a skeleton, they're basically ''all'' muscle.</ref>) 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. <ref>It wasn't. It was food poisoning from a misunderstood recipe.</ref>

* 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.<ref>''With gusto.'' Without a skeleton, they're basically ''all'' muscle.</ref>) 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.<ref>It wasn't. It was food poisoning from a misunderstood recipe.</ref>

* 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 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 [[X Wing Series|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.

* In [[X Wing Series|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.

** 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.

{{quote|'''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 -"

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* The [[Lois McMaster Bujold|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 {{spoiler|the Descolada was engineered to specifically be able to adapt to different genetic codes, and that the virus may be semi-intelligent itself}}.

* 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 ''[[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.

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* 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).

** [[Doctor Who/Recap/S32/E10 The Girl Who Waited|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.

* ''[[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.

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** 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.

** 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.

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=== [[Anime]] ===

* The planet Namek in ''[[Dragon Ball]] 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 [[~~Lampshade|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.

** There's also a scene that [[lampshade]]s 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]] ===

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=== [[Film]] ===

* 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.

* 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 [[Heavyworlder|stocky]]. Otherwise it really doesn't come up.

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** Also [[All There in the Manual|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.

** 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 [[Beneath the Earth|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...)

* In ''[[Final Fantasy IV]]'', the party goes from the surface of the planet to the [[Beneath the Earth|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 [[Psychic Powers|Psionic powers]] to support their atrophied body mass... and some just seem to do fine without any help. [[Gameplay and Story Segregation|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.