History NoBiochemicalBarriers / CrossSpeciesDisease

* 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 or other animal vectors. Smallpox and anthrax are of bovine origin, influenza of avian and porcine stock, the common cold may be from horses, Ebola is from bats, HIV a variant of SIV from African green monkeys, etc. The only ones that don't show strong relationships with livestock or other animals are [=STDs=], with the exception of HIV. 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 ludicrous in an alien. The Wuhan Coronavirus first appeared in humans who visited a certain food market, and has been suggested to have come from snakes or bats.

* In one of episode of ''LightNovel/HaiyoreNyarkoSan'', the HumanoidAlien shapeshifter Nyarko falls ill due of the cold with Hasta elaborating that they don't have any immunity to Earth viruses (except Cuuko, who can adjust her own body temperature), leading to a brief discussion of how aliens have the most random {{Weaksauce Weakness}}es. When she gets accidentally teleported in while the rest of the cast are fighting against the invading Mi-go, they try to use her cold to infect them as well before their leader reminds his troops that they were already vaccinated after they had to leave Earth the last time they were there.

* 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 or other animal vectors. Smallpox and anthrax are of bovine origin, influenza of avian and porcine stock, the common cold may be from horses, Ebola is from bats, HIV a variant of SIV from African green monkeys, etc. The only ones that don't show strong relationships with livestock or other animals are [=STDs=], with the exception of HIV. 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 ludicrous in an alien. The Wuhan Coronavirus first appeared in humans who visited a certain food market, and has been theorized to have come from snakes.

* 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 or other animal vectors. Smallpox and anthrax are of bovine origin, influenza of avian and porcine stock, the common cold may be from horses, Ebola is from bats, HIV a variant of SIV from African green monkeys, etc. The only ones that don't show strong relationships with livestock or other animals are [=STDs=], with the exception of HIV. 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 ludicrous in an alien.

[[folder:Films ― Live-Action]]

* ''Series/DoctorWho'': [[Recap/DoctorWhoS32E10TheGirlWhoWaited "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.

** A popular plot device, possibly because, as per TNG: "The Chase," most if not all of the galaxy's humanoid species [[{{Panspermia}} share a common ancestry]]. It's still weird given the physiological differences (take the Vulcan/Romulan species, which has copper-based blood).

* ''Wiki/SCPFoundation'': Not a cross species disease, but cross species medication. In the GOI format article about The Choir Below, an alien race resembling bizarre singing worms, a person comments that they once saved one of them from an anaphylactic reaction by using EpiPen, and then [[LampshadeHanging lampshades]] how surprising it is that it worked.

* ''WebOriginal/SCPFoundation'': Not a cross species disease, but cross species medication. In the GOI format article about The Choir Below, an alien race resembling bizarre singing worms, a person comments that they once saved one of them from an anaphylactic reaction by using EpiPen, and then [[LampshadeHanging lampshades]] how surprising it is that it worked.

** The Flood can infest any creature with enough biomass and individual sapience to support the parasite. This specifically excludes Hunters and Drones (being TheWormThatWalks and a giant termite, respectively), though it can still use them as raw biomass.

[[folder:Films -- Live-Action]]

* ''Series/DoctorWho'': "[[Recap/DoctorWhoS32E10TheGirlWhoWaited 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.

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* ''WesternAnimation/StarWarsTheCloneWars'': [[Recap/StarWarsTheCloneWarsS1E17BlueShadowVirus "Blue Shadow Virus"]]/[[Recap/StarWarsTheCloneWarsS1E18MysteryOfAThousandMoons "Mystery of a Thousand Moons"]] have the titular virus, 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.

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** 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. He was rightfully laughed at for that hypothesis. Ironically, he may have been right about the "protect from falling germs" idea, just mistaken about where they came from: Earth's atmosphere is saturated with bacteria that rain down on us 24-7, but they're ''Earth'' bacteria dispersed by wind.

[[folder:Anime and Manga]]
* In ''Manga/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. While Goku is an alien himself, the Saiyan biochemistry is identical to a human being's as far as [[BigEater nutrition]] and [[HalfHumanHybrid reproduction]] are concerned.

* Taken UpToEleven in episode 4 of ''Anime/SpaceDandy'', with a [[ZombieApocalypse zombie virus]] that infects every single species in the galaxy, including humans, all kinds of aliens and ''robots.''

* The Franchise/GreenLantern {{Arc}} ''Comicbook/SinestroCorpsWar'' had a virus called [[Characters/GLSinestroCorps Despotellis]] capable of killing any species, even ones normally resistant to disease. This being a superhero universe, Despotellis is ''sentient''.

[[folder:Film — Live-Action]]

* In ''Literature/TheAndromedaStrain'', the alien "virus"[[note]]Actually, a weird crystalline lifeform.[[/note]] [[ContrivedCoincidence by a freak coincidence]] is able to thrive in the exact set of condition that occur in the human bloodstream. However, it is evolving so fast that it becomes unable to do so barely in a couple of weeks, eventually making itself entirely harmless. In fact, its last fatality was a fighter pilot who dies not from infection, [[NotTheFallThatKillsYou but from his jet crashing]] after an intermediate form of the virus consumes all the plastic seals in his jet.

* ''Series/DoctorWho'':
** [[Recap/DoctorWhoS32E10TheGirlWhoWaited "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.

** 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...[[NightmareFuel odd]], at least the first time. (This was retconned, as the episode in question was uniformly hated by the writers.) 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 Reetou. 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.

* The Life-Eater virus in ''TabletopGame/{{Warhammer 40000}}'' fluff can scour entire planets clear of life in minutes, no matter what kind of life inhabits said planet. Even Eldar (with [[ArtisticLicenseBiology quintuple helix DNA]]) and Tyranids (which may have local DNA salvaged from corpses, but the race as a whole comes from a ''different galaxy''). Some fluff has taken an intelligent turn, and implied that it's a sort of [[GreyGoo nano weapon]].

* In ''WesternAnimation/{{Ben 10}}'', each of Ben's aliens manifested different symptoms of his human cold; Heatblast, for example, gets his powers reversed from [[PlayingWithFire fire]] to [[AnIcePerson ice]].
* ''WesternAnimation/JusticeLeague'': One episode had a rogue Amazon [[{{Gendercide}} 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 allergen, even though allergies don't actually work like that. However, given that part of the creation process involves the use of crushed rubies as a component, most likely the disease is as much magical as it is biological.

** 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 cause 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 - or simply grow in an inconvenient location (e.g. on those nice heart valve flaps...). 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) "eating" parts of the host. Viruses "eat" cells only in a far less literal sense, requiring a certain degree of DNA compatibility to replicate.

* Brought up and then averted in an approved doujin sidestory to ''Anime/DanceInTheVampireBund''. Not only are vampires immune to all human diseases (Except for tooth decay, which isn't communicable), some vampires actively seek out the blood of the ill, as the disease changes the flavor.

* ''VisualNovel/HigurashiWhenTheyCry'': It is speculated that the parasite behind the Hinamizawa Syndrome actually came from another planet. Then again [[spoiler: the person who said that is also the one who wanted to turn it into a bio-weapon, so it's questionable how trustworthy this is]].

* The Franchise/GreenLantern {{Arc}} ''Comicbook/SinestroCorpsWar'' had a virus called [[Characters/GLSinestroCorps Despotellis]] capable of killing more than one species, even ones normally resistant to disease. Probably a JustifiedTrope since Despotellis is ''sentient''.

** Somewhat justified by a suggestion that the Xenomorphs were created as either a bioweapon or a terraforming construct (which is 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...
** Ripley in ''A:R'' was [[spoiler: only the latest, successful attempt at cloning a human/Xenomorph hybrid]]. There's the memorable (and oft-referenced) scene where [[spoiler: she burns the whole laboratory containing her failed predecessors after one of them begs for a MercyKill]].

* Played quite straight and then subverted in ''Literature/TheAndromedaStrain'', where the alien "virus"[[note]]Actually, a weird crystalline lifeform.[[/note]] [[ContrivedCoincidence by a freak coincidence]] is able to thrive in the exact set of condition that occur in the human bloodstream. However, it is evolving so fast that it becomes unable to do so barely in a couple of weeks, eventually making itself entirely harmless. In fact, its last fatality was a fighter pilot who dies not from infection, [[NotTheFallThatKillsYou but from his jet crashing]] after an intermediate form of the virus consumes all the plastic seals in his jet.

* 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.
* Creator/IsaacAsimov's ''Literature/DavidStarrSpaceRanger'': A series of poisonings occurs in people who ate Mars-grown food. A (human) Martian scientist says it could have been caused by the local bacteria. [[spoiler:{{Subverted|Trope}} later; he was the one behind the poisonings, and the protagonist realized he had been telling a deliberate lie.]]
* In ''Literature/EncounterWithTiber'', its averted, with Tiberians being too different for earth organisms to attack them.

* Averted in ''Literature/TheFlightEngineer'' between humans and [[InsectoidAliens Fibians]], though the human characters do check to make sure before [[spoiler:meeting with the Fibian central government]].

** 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. In fact, per the BackStory, the same plague devastated the population of three worlds and their space forces several times.

** The planet Grayson is a DeathWorld full of heavy metals which forces them to do all their agriculture either under protective domes or on orbital farms. 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.
** 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.
* Creator/LarryNiven's ''Literature/KnownSpace'' series:
** Avoided in the short story "Literature/MadnessHasItsPlace". 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 ''Literature/{{Ringworld}}'' 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 [[spoiler:the humanoid inhabitants of the Ringworld are all distantly related to each other, and to Earthlings as well]].
** [[ScienceMarchesOn 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 [[spoiler: Pak]] simply didn't put any pathogens there, V.D. included.

* Averted in James White's ''Literature/SectorGeneral'' book series about an interstellar hospital. (This was someone who would HandWave most sci-fi technologies, but think long and hard about how to perform CPR on a six-foot-long, silver-furred, sentient caterpillar. [[note]]''With gusto.'' Without a skeleton, they're ''all'' muscle.[[/note]]) 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. [[spoiler:It wasn't. It was food poisoning from a misunderstood recipe.]]
* 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...
* In the ''Literature/SpeakerForTheDead'' 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]].

* ''Franchise/StarWarsLegends'':
** In ''[[Comicbook/XWingSeries The Krytos Trap]]'', Imperial scientists engineer a biological weapon (the Krytos Virus) 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 means that it quickly becomes less lethal.
** In the same series, Corran recounts the tale of bringing one of his non-human classmates (whom he had a heavy-duty crush on) to the Academy ball, and then going back to his place for mind-numbingly good sex, only to find out the hard way the morning after that he apparently had a severe allergy... to ''her fur''. And ''she'' had a bad medical reaction to his ''sweat''. He would later describe the effect that each experienced as like having a full-body sunburn.
** In ''Literature/NewJediOrder'', an engineered bioweapon is developed for use against the Yuuzhan Vong, who are ''extra-galactic''--there are significant amounts of DNA they have that no known organism in the galaxy far, far away does.
** Naturally, this comes up often in the ''[=MedStar=] Duology'', a pair of ''Star Wars'' novels that serve as a medical drama.
*** Bacta seems to work on anything, but other medicines and treatments vary from species to species.
---->'''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--\\
'''Barriss:''' 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, acting as anything from medicine to a nutrient to an incredibly potent drug, and briefly raises Force-sensitives into something very like [[AGodAmI godhood]]. The Republic and the Separatists are fighting a pitched and prolonged battle across an otherwise useless backwater world for the sake of the stuff. 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, [[ShaggyDogStory bota has mutated itself into uselessness]].
** In ''Literature/GalaxyOfFear: The Planet Plague'', there's a medical research station in an outpost on Gobindi, which is covered in ruins left by a now-lost civilization which studied diseases extensively. It also has a BlobMonster infestation, and said monsters can infect humans [[spoiler: [[TheVirus and turn them into blobs]]]]. It's found that the Imperials found a virus native to Gobindi, have been modifying it, and are now turning it loose on the outpost, looking to find out which species besides humans are affected. It's also darkly hinted in TheStinger that the native Gobindians were killed by some disease even they couldn't handle and the BigBad has a sample, but [[WhatHappenedToTheMouse this isn't followed up on,]] so it probably didn't work.

** 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 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.

* [[PlayingWithATrope Played with]] in the ''Series/AgentsOfSHIELD'' episode "FZZT". A ''[[Film/TheAvengers2012 Chitauri]]'' helmet infects some firemen [[spoiler: and Jemma]] with an alien virus that makes an electric burst blow a hole in people's heads. However, they figure out that the Chitauri itself was immune to the virus, and was just a carrier. It's even theorized to be the Chitauri equivalent of a cold.

* Averted in an episode of ''Series/{{Battlestar Galactica|2003}}''. 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 [[spoiler: a probe from an Earth that had been populated by Cylons (the Thirteenth Tribe of Kobol). So in reality they contracted it from an earlier form of themselves]].
** Unfortunately played straight when [[spoiler: the fleet reaches a second planet they dub Earth. Yes, it's our Earth, and humans have magically evolved there too. However, that's the ''least'' of the problems both scientific and dramatic with the finale..]].
* In ''Series/BattlestarGalactica1978'', two Viper pilots pick up an alien disease on a mission, and skip decontamination to make it to a bachelor party. The resulting disease ravages the fleet. How, exactly, they picked up an alien disease on an apparently lifeless moon, well …

** The Doctor himself has repeatedly had to fend off ''human'' doctors' attempts to treat him with human-specific medications, which would be harmful or at least ineffectual to him.
** [[Recap/DoctorWhoS31E8TheHungryEarth "The Hungry Earth"]]/[[Recap/DoctorWhoS31E9ColdBlood "Cold Blood"]]: A human is stung by the venomous tongue of a Silurian warrior, and begins 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).

** [[Recap/DoctorWhoS36E7ThePyramidAtTheEndOfTheWorld "The Pyramid at the End of the World"]] averts it: An incredibly deadly bacteria is accidentally created in a lab, and the Doctor goes to deal with it. He's immune because he's not from Earth. [[spoiler:Nardole, on the other hand, is affected because, thanks to the Doctor rebuilding him, he now qualifies as "human enough" to get sick. However, he's not as badly affected as a normal human.]]

** Possibly justified by the fact that, as per TNG: "The Chase," most if not all of the galaxy's humanoid species [[{{Panspermia}} share a common ancestry]]. It's still weird given the physiological differences (take the Vulcan/Romulan species, which has copper-based blood).
*** Organisms with apparently diverse physiology can actually very commonly contract disease from each other. Vulcans and Romulans have copper-based blood, but said blood transport differences do not stop their lung cells from contracting very similar viruses.
*** Star Trek also averted this trope on Voyager when different power requirements for a damaged robot was compared to giving a Bolian a blood transfusion from a Vulcan; the doctor agrees and mentions that this would kill the unfortunate Bolian.

*** In the original series episode "Miri," Kirk, Spock, and several human members of the crew get infected by a rage inducing disease. Spock is immune to the diease itself but [[TyphoidMary thinks he carries the disease]] so would be stuck on the planet in a self imposed quarantine unless a cure is found.

* {{Zigzagg|ingTrope}}ed in ''TabletopGame/BleakWorld'' where some of the aliens are horribly damaged by human diseases, but some other Aliens are immune to them completely.
* Played with in the Classic ''TabletopGame/DungeonsAndDragons'' 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.
** 3rd Edition's disease rules make no distinction based on character species. Unless a race or class is marked as immune to disease, it can contract said illness. Granted, many diseases in the various rulebooks are magical in nature so [[AWizardDidIt the normal rules may not apply]].

** Nurgle's Rot is another disease that can effect all life. This one is a bit more [[JustifiedTrope justified]] because the disease is created by '''[[{{Plaguemaster}} the god of plague]]''', and is more of a [[MysticalPlague magical/psychic phenomenon]] than a physical one. It is not immediately fatal, but slowly rots the body and soul to drive one to despair while it creates a prolonged death (both of which empower Nurgle). Even worse, as it runs its course, [[TheCorruption it corrupts the souls of its victims]] into [[TheLegionsOfHell Plaguebearers]].
** Justified by all sentient species being artificial rather than naturally evolved: the Old Ones and Necrons (who may be closely related in themselves) engineered most sentient life in the universe, though they used different base 'materials' for different species. Anything that attacks those common engineered parts of sentient biology could logically infect more or less anything in the setting but the Tyranids... and the hat of the Tyranids is 'adapts to be compatible with everything' including diseases, so they're logically on the list too.

* Averted in ''VideoGame/FTLFasterThanLight''. One {{Random Encounter|s}} has a plague on a human world. If you have any alien crew members they can help out without fear of contracting it.

* Discreetly averted as a GeniusBonus in ''VisualNovel/HatofulBoyfriend''. The heroine mentions that just talking to DeadlyDoctor Iwamine Shuu can catch you all sorts of nasty diseases; the examples she cites are [[ShownTheirWork actual diseases]] that can affect both birds and humans.
** Avian influenza, another disease known to make a species jump, is involved in the backstory of birds being sapient, and a fictional virus in the "Bad Boys Love" route specifically has a very different effect on humans than on birds.

** 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, 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 [[spoiler:in the best ending of the Rannoch quest, Tali indicates that the geth have started working with the quarians suits to boost their immunity, so soon they'll be able to walk around their home planet without suits]].
*** Actually, Quarians get infections all the time precisely because of their less-than-robust immune systems being also compromised by centuries of existence on ships, forcing them into their suits virtually permanently. Information from Reegar and the codex explains that Quarian infantry tend to avoid large-scale engagements. Logistical problems would prevent them getting the food they need, and antibiotics wouldn't get to the casualties that need it to fight injury-induced infection. Reegar takes a hit from a Geth weapon in Tali's recruitment mission in Mass Effect 2, and says he is now swimming in antibiotics to prevent infection from the wound.
** 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.

* The Zerg Hyperevolutionary Virus from ''VideoGame/StarCraft'' is able to infect any alien it comes in contact with. It's also usually paired with a neural parasite. This is Justified by having the virus, as per namesake, evolve rapidly (about a million times more in a week than humans ever have in two million years). And the assumption that everything has DNA to infect. However, it doesn't work on Protoss due to their PsychicPowers supercharging their immune systems.

* ''VideoGame/XenobladeChroniclesX'': While it is a DiscussedTrope by the humans when it comes to allowing friendly xenos into NLA and they are scanned for potentially harmful pathogens before being let in, there is a grand total of one case of disease affecting the humans of NLA on planet Mira, and its cause is quickly discovered and dealt with. There are a couple of other cases though, with one of them being of vital plot importance: [[spoiler:the Orphean's "Ovah" is eventually revealed to be a symbiotic virus, and while it's harmless to most species, it does begin getting into some of the native Miran populations, which disturbs the Orpheans greatly; while the plot-important one is the fact that human DNA is extremely toxic to the Ganglion, which as it turns out is the whole reason they wanted to exterminate humanity in the first place]].

[[folder:Webcomics]]
* Justified and subverted in ''Webcomic/{{Miamaska}}''; pathogens carried by otherworlders (human) are perfectly compatible with Alodian locals (also human), who have no exposure or immunities to them, resulting in devastating plagues. Consequently, most Alodians have a "kill on sight and burn the body" policy regarding otherworlders. It's also a two-way street; Amity got sick drinking local raw milk because her system has no immunity to Alodian microbes.
* Not disease, but medicine - ''Webcomic/SchlockMercenary'' averts this nicely by having characters have med-kits in their [[TankGoodness 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, since they can be remotely programmed or updated with the specific needs of their host body, although a few occasions with extra rare species not in the database require manual configuration by the doctor.

[[folder:Web Original]]
* {{Discussed|Trope}} several times in ''Machinima/FreemansMind'', as Gordon really hopes that it's an AvertedTrope and that alien DNA is too different for their diseases to take hold.

[[folder:Western Animation]]
* In ''WesternAnimation/{{Ben 10}}'', each of Ben's aliens manifested different symptoms of his human cold; Heatblast, for example, gets his powers reversed from [[PlayingWithFire fire]] to [[AnIcePerson ice]].
* ''WesternAnimation/InvaderZim'':
** Somewhat averted when lice broke out and 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 harmful to him in some way. 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.
* ''WesternAnimation/JusticeLeague'':
** One episode had a rogue Amazon [[{{Gendercide}} 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 allergen, even though allergies don't actually work like that. However, given that part of the creation process involves the use of crushed rubies as a component, most likely the disease is as much magical as it is biological.
** Played with when Kilowog crash lands on Earth. J'onn is able to treat him to an extent because his own physiology is fairly similar to Kilowog's, but when Flash asks how much he has to guess to fill in the gaps the martian says he doesn't want to answer that question. Still, the drugs he uses all work just fine.
* In one episode of ''WesternAnimation/StarTrekTheAnimatedSeries'', the crew visits a planet with sentient plants that were nearly wiped out by a gram-negative bacteria from a ''human'' visitor some years ago.
* ''WesternAnimation/StarWarsTheCloneWars'': [[Recap/StarWarsTheCloneWarsS1E17BlueShadowVirus "Blue Shadow Virus"]]/[[Recap/StarWarsTheCloneWarsS1E18MysteryOfAThousandMoons "Mystery of a Thousand Moons"]] have the titular virus, 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.
* In ''WesternAnimation/YoungJustice'', M'gann the Martian appears to catch a human flu, prompting Robin to [[ShoutOut mention]] ''Literature/TheWarOfTheWorlds'', as well as mentioning how unlikely that would be. It's ultimately an aversion, since her "illness" was actually the result of [[spoiler:the power-leeching villain Parasite]].
[[/folder]]

[[folder: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 or other animal vectors. Smallpox and anthrax are of bovine origin, influenza of avian and porcine stock, the common cold may be from horses, Ebola is from bats, HIV a variant of SIV from African green monkeys, etc. The only ones that don't show strong relationships with livestock or other animals are [=STDs=], with the exception of HIV. 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 ludicrous 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 cause 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 - or simply grow in an inconvenient location (e.g. on those nice heart valve flaps...). 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) "eating" parts of the host. Viruses "eat" cells only in a far less literal sense, requiring a certain degree of DNA compatibility to replicate.
** 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. He was rightfully laughed at for that hypothesis. Ironically, he may have been right about the "protect from falling germs" idea, just mistaken about where they came from: Earth's atmosphere is saturated with bacteria that rain down on us 24-7, but they're ''Earth'' bacteria dispersed by wind.
[[/folder]]

[[folder:Film]]

* Played quite straight and then subverted in ''Literature/TheAndromedaStrain'', where the alien "virus"[[note]]Actually, a weird crystalline lifeform.[[/note]] [[ContrivedCoincidence by a freak coincidence]] is able to thrive in the exact set of condition that occur in the human bloodstream. However, it is evolving so fast that it becomes unable to do so barely in a couple of weeks, eventually making itself entirely harmless. In fact, its last fatality was a fighter pilot who dies not from infection, [[NotTheFallThatKillsYou but from his jet crashing]] after an intermediate form of the virus consumes all the plastic seals in his jet.

* Played with in the Classic [[TabletopGame/DungeonsAndDragons 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.

** 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 if the virus was able to adapt it would still need very specific circumstances that are likely unique to Earth.
*** Incorrect - it is more likely that some form of DNA will exist, given the ideal conditions for complex biochemistry to occur. We like to insist that if life came from another world, it simply MUST be different in the extreme, because 'alien' should mean it. We like the idea of ammonia-based ecologies or methane-breathing organisms when this is simply a much more difficult scenario for early life to have thrived. However given the limited number of conditions in which life will easily flourish, it is far more likely an alien form of life will have arisen on a world with liquid water, with survivable levels of radiation and similar conditions for the beginning of biochemistry. The biggest differences are likely to be metabolic, the means they generate energy for their bodies. This may involve some process other than photosynthesis, which would make it more likely an alien ecosystem has less oxygen.
*** Even if a virus found a DNA-using host cell, and had the right biochemical tricks to enter (viruses normally use some specific proteins on their surface to interact with specific proteins on the host cell in order to enter), and were able to insert their DNA into the host cell's DNA (many viruses don't care where they insert, and have their own support sequences to get themselves copied), it would then almost certainly sit there and do absolutely nothing because the sequences that tell the host cell to make proteins based on the viral genes are extremely unlikely to work on the alien host. Even if they do, the alien host will likely use a completely different coding system for amino acids, and likely some different amino acids as well, making the resulting proteins worthless junk.
*** As pointed out above, we have only Earth disease-causing microorganisms to go on. It is even money whether an extraterrestrial life form could prove infectious. A bacteria need only be able to metabolise some small aspect of our tissues or blood to be extremely dangerous! A virus may not be able to infect terrestrial cells - but it really depends what TYPE of cell those viruses evolved to parasitise. A similar-enough DNA strand could still be infected, and don't forget many viruses cause the host cell itself to use its own transcription and translation machinery to replicate. Even if 'alien' proteins result...what if said proteins are highly toxic? What if they turn out to encode a neurotoxin, or a haemolytic enzyme? Or the junk protein was of a different chirality, causing fatal immune reactions? This kind of science cannot be based in skepticism, like all science. When we have an alien microbe...we'll find out.

** 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. In fact, per the BackStory, the same plague devastated the population of three worlds and their space forces several times.

* Creator/IsaacAsimov's ''Literature/DavidStarrSpaceRanger'': A series of poisonings occurs in people who ate Mars-grown food. A (human) Martian scientist says it could have been caused by the local bacteria. [[spoiler:{{Subverted|Trope}} later; he was the one behind the poisonings, and the protagonist realized he had been telling a deliberate lie.]]

* ''Literature/LuckyStarr'': 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. [[spoiler:Subverted later, when it turns out he was the BigBad and was telling a deliberate lie.]]

** The Pinthi virus seems to kill anyone and anything infected with it. Perhaps justified in that the virus was intentionally engineered to be a biological weapon that breaks down organic tissue (frequent reference is made to victims ''liquefying''). The virus is also intelligent, and capable of some level of self-modification, though by the time of the game it has decided it doesn't want to kill others and tires to avoid infecting people if at all possible.

* In ''WesternAnimation/YoungJustice'', M'gann the Martian appears to catch a human flu, prompting Robin to [[ShoutOut mention]] ''Literature/TheWarOfTheWorlds'', as well as mentioning how unlikely that would be. It's ultimately an aversion, since it was actually a power-leaching villain responsible.

* Justified and subverted in ''Webcomic/{{Miamaska}}''; pathogens carried by otherworlders (human) are perfectly compatible with Alodian locals (also human), who have no exposure or immunities to them, resulting in devastating plagues. Consequently, most Alodians have a "kill on sight and burn the body" policy regarding otherworlders. It's also a two-way street; Amity got sick drinking local raw milk during her amnesia period because her system has no immunity to Alodian microbes.