"Stay back! The plants kill!"—Sign outside of Vault 22, Fallout: New Vegas
The Festering Fungus is a fictional lifeform usually based on a mold or a lichen, know for its rapid growth rate and destructive tendancies.
The effects of the fungus will vary. The Lighter and Softer versions will usually just cause structural damage to buildings, cars and trees, while the nastier versions start growing on people, resulting in all sorts of Body Horror. Ingesting the spores of this lifeform is to be avoided at all cost.
This type of lifeform tends to make for a good Monster of the Week. After all, it's usually nonsentient, so you can finish it off for good without feeling guilty about it. The protagonists will usually find a way to stop it by the end of the episode, often with the aid of some fantastical fungicide or by dropping the temperature so that it can't grow.
In a comedy series, expect it to have crawled out of someone's refrigerator. See also: Alien Kudzu, Fungus Humongous, The Virus. Mold-based variats may overlap with Blob Monster Muck Monster, or Meat Moss. People infected with one of these may become Viral Humanoids.
- In the episode "The Sound of Rust" of Mushishi, A fungus-like rust infects a girl's vocal chords, and whenever she speaks, rust spreads like lichen across her village, and even on the skin of the other villagers. The girl becomes a pariah to her people due to her connection with the plague.
- Episode 229 of Naruto Shippuden titled, "Eat or Die! Mushrooms from Hell!" had multicolored mushrooms called "Moulding Mushrooms" overtake the ship on which the team is sailing. Upon being eaten, the mushrooms sprout from the eater's skin and influence their actions.
- Spirou and Fantasio: The Count of Champignac is a mycologist who uses a very fast-growing mutated mold as a bio-weapon. In one instance, said mold destroyed a city-sized secret base in just a few minutes.
- Morrigan Lugus from Supergod is a superhuman entity made up of 3 unlucky astronauts fused together with alien fungus to form a sentient mycological supercomputer. The facility it's kept in has to be cleaned over with bleach almost constantly to prevent mushrooms from growing everywhere, and before the science team started taking anti-fungal medications, several of them died and during the autopsy it was discovered that their lungs were filled with fungal growths that looked like deformed human faces.
- The mobile green slime which threatened the International Space Station in one segment of The Secret Return of Alex Mack.
- The king in Super Mario Bros. the movie became this due to de-evolution.
- Matango, a 1963 Japanese horror film based loosely on "The Voice in the Night" (see below).
- One short horror film had a man infected with an unknown fungus that infected and grew on everything he touched, including his blood which grew into more fungus. It eventually takes over his body, turning him into a monster and killing his best friend who comes over to his house out of concern.
- The Ur Example may be the 1907 short story "The Voice in the Night" by William Hope Hodgson.
- In a similar vein, the 1923 story "Fungus Isle" by Philip M. Fisher.
- The often-mentioned, never fully-explained "mold demons" from The Dresden Files.
- Expedition Venus by Hugh Walters has Earth threatened by a fast-growing mould brought back to Earth by an unmanned space probe. Includes some disturbing descriptions of what happens when it grows on people. (As the trope description says, it's a Monster of the Week—this is the fifth novel in a series—and the heroes sort it out by the end of the novel.)
- The title city of Ambergris is prone to this, and positively swimming in it by the time of Finch. Related to rule by the Graycaps, who are themselves a spore-based lifeform.
- Robert Westall's Urn Burial has harka, a fungal mould created by the Attock which primarily infects the females of species and kills young developing in the womb. In adults it just eats away their flesh and strips them down to the bone. The only way of killing it is to teleport the infected into stars.
- The aforementioned William Hope Hodgson story "The Voice in the Night" was made into an episode of the TV show Suspicion in 1958.
- These things were in the spotlight of several of The X-Files episodes, most notably, "El Mundo Gira".
- The Pokémon Parasect is a tragic victim of one of these. Its pre-evolution, Paras, has a symbiotic relationship with the Cordyceps-like fungus growing on its head, but when it evolves, the fungus takes over its body completely, leaving it a blank-eyed zombie bug.
- Nethack has a number of mold mobs, palette-swapped by how they hurt you.
- Phazon, from Metroid Prime, is most often encountered in the form of a highly radioactive, rapidly growing, seemingly sentient fungus that either kills or corrupts anyone or anything that has contact with it for too long.
- Fallout: New Vegas had these as the result of experimenting with genetically altered spores. They're found in Vault 22 and in the DLC, Zion Valley and Big M.T., where they were originally created. Worst of all is how you can find "Runts" in Vault 22 as well as baby carriages covered in spores.
- Cordyceps are a type of fungi that infects living creatures (mostly insects and other arthropods). They grow inside the infected bug until it kills it, at which point the fungus bursts out of the host's exoskeleton and flowers, spreading its spores. Even creepier is that some species of cordyceps actually affect the host's behavior. For instance, one species infects ants. The infected ants gain a compulsion to climb upward as far as they can before they die, which allows the cordyceps to spread its spores further. These species are commonly referred to as the "zombie fungus".
- The really fun part is that it is commonly eaten by humans, and is even used in many medications such as the anti-rejection drug Neoral.