History NoBiochemicalBarriers / CrossSpeciesDisease

[[folder:Fan Works]]

* ''{{Series/Tracker}}'' had a fugitive who intentionally infected himself back home and was an assassin of sorts. His natural form could infect and kill another alien just by touching them. In human form, he was caught in a MateOrDie situation, needing to have sex with human women to pass on the virus before it built up enough in him to kill him. Includes a bit of FridgeHorror given how close he came to infecting Mel, who was later revealed to be a HalfHumanHybrid. She might have died faster.

* Purposely {{averted|Trope}} in ''VideoGame/TheDig'', where the characters go out of their way to point out that despite the surprise of the breathable atmosphere on Cocytus, it's extremely unlikely that any possible airborne microbes could have evolved to be able to affect humans in particular. [[spoiler:This doesn't stop the characters from falling prey to a disease of a [[FantasticDrug different sort]] later on.]]

* Purposely [[AvertedTrope averted]] in ''VideoGame/TheDig'', where the characters go out of their way to point out that despite the surprise of the breathable atmosphere on Cocytus, it's extremely unlikely that any possible airborne microbes could have evolved to be able to affect humans in particular. [[spoiler:This doesn't stop the characters from falling prey to a disease of a [[FantasticDrug different]] [[PsychoSerum sort]] later on.]]

* 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. However, it doesn't work on Protoss due to their PsychicPowers supercharging their immune systems.

* In ''Franchise/TheElderScrolls'', diseases tend to jump across the species barrier between humans, elves, and beastfolk, with only the Argonians being largely immune to most diseases. However, most diseases have some form of supernatural element to them, as they are often created by the [[OurGodsAreDifferent Daedric Prince Peryite]], who has pestilence and disease as part of his sphere of influence. One particularly notable instance was the Thrassian Plague,a supernatural disease created by the amphibian species known as the Sload, which wiped half of the total population on Tamriel. This resulted in the ''only'' known instance of [[EnemyMine every surviving race on the continent putting aside their differences to attack the Sload in retaliation.]]

** Before SARS-CoV-2, the SARS-CoV-1 virus crossed from bats to civets and then to humans and several other mammals. There is also the MERS-CoV virus, which crossed from bats to camels and then to humans.

* In the ''[[Literature/{{Hainish}} Hainish Cycle]]'' by Ursula K. Le Guin; measles is described as a cross-species disease. In ''TheDispossessed''; Shevek needs to get vaccinated before visiting the planet of Urras, where measles is apparently endemic, while it had been kept out of his home world of Anarres by strict quarantine. Le Guin justified this by the Cetians and all other intelligent species being HumanAliens that were spread throughout space by Hainish space travel a few million years ago; so they would be vulnerable to the Earth humans in the setting bringing measles with them.

* In the ''[[Literature/{{Hainish}} Hainish Cycle]]'' by Ursula K. Le Guin; measles is described as a cross-species disease. The character of Shevek needs to get vaccinated before visiting the planet of Urras, where it is apparently endemic, while it had been kept out of his home world of Anarres by strict quarantine. Le Guin justified this by the Cetians and all other intelligent species being HumanAliens that were spread throughout space by Hainish space travel a few million years ago; so they would be vulnerable to the Earth humans in the setting bringing measles with them.

* In the ''[[Literature/{{Hainish}} Hainish Cycle]]'' by Ursula K. Le Guin; measles is described as a cross-species disease. The character of Shevek needs to get vaccinated before visiting the planet of Urras, where it is apparently endemic, while it had been eradicated on his home of Anarres by strict quarantine. Le Guin justified this by the Cetians and all other intelligent species being HumanAliens that were spread throughout space by Hainish space travel a few million years ago. However, [[ScienceMarchesOn more recent research]] shows that measles virus only evolved on Earth in the last couple of thousand years.

* In the ''[[Literature/Hainish Hainish Cycle]]'' by Ursula K. Le Guin; measles is described as a cross-species disease. The character of Shevek needs to get vaccinated before visiting the planet of Urras, where it is apparently endemic, while it had been eradicated on his home of Anarres by strict quarantine. Le Guin justified this by the Cetians and all other intelligent species being HumanAliens that were spread throughout space by Hainish space travel a few million years ago. However, [[ScienceMarchesOn more recent research]] shows that measles virus only evolved on Earth in the last couple of thousand years.

* Leprosy, a disease that mainly infects humans, strangely can also be carried by nine-banded armadillos.

* Your MissionControl in ''VideoGame/SystemShock2'' tells you that a life form found on Tau Ceti V managed to employ a parasitic process to infect and nearly wipe out the entirely human crew that arrived to explore it. [[spoiler:This turns out to be a subversion, though, as the life found there actually originated on Earth -- and your MissionControl ''created it'']].

* Your MissionControl in ''VideoGame/SystemShock2'' tells you that a life form found on Tau Ceti V managed to employ a parasitic process infect and nearly wipe out the entirely human crew that arrived to explore it. [[spoiler:This turns out to be a subversion, though, as the life found there actually originated on Earth -- and your MissionControl ''created it'']].

* Your MissionControl in ''VideoGame/SystemShock2'' tells you that a life form found on Tau Ceti V managed to employ a parasitic process infect and nearly wipe out the entirely human crew that arrived to explore it. [[spoiler:This turns out to be an aversion, though, as the life found there actually originated on Earth -- and your MissionControl ''created it'']].

* In one of episode of ''LightNovel/NyarukoCrawlingWithLove'', the {{Humanoid Alien|s}} shapeshifter Nyarko falls ill due to 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 are vaccinated after they had to leave Earth in the past for this exact reason.

* In one of episode of ''LightNovel/NyarukoCrawlingWithLove'', the HumanoidAlien shapeshifter Nyarko falls ill due to 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 are vaccinated after they had to leave Earth in the past for this exact reason.

* The Franchise/GreenLantern {{Arc}} ''ComicBook/SinestroCorpsWar'' has 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''.
* Played reasonably straight in Creator/AlanMoore's ''ComicBook/TopTen'' with Sudden Traumatic Organic Rapid Mutation Syndrome or S.T.O.R.M.S. This unpleasant sexually transmitted disease mutates sufferers into a new, invariably [[BodyHorror non-viable lifeform]] (and in one case [[AGodAmI a god]]). Interestingly it is said to be the result of a city full of [[FantasyKitchenSink humans, super human, mutants, aliens, Robots, magical creatures and all the variations from alternate universes]] all interacting sexually with each other.

* The absence of such barriers is an important plot point in ''LightNovel/InvadersOfTheRokujyouma''. When Koutarou is transported to Forthorthe 2000 years in the past, he's exposed to a virus that debilitates the Forthortheans but only causes minor symptoms in him and Clan (a modern Forthorthean). This is not because the former is human, but because the two of them have genes for resistance to the virus, which Clan uses to develop gene therapy to treat the virus. The odds of Earthlings and Forthortheans having genomes similar enough for this to happen is {{lampshade|Hanging}}d, with speculation that the two species are one and the same and were deliberately placed on their respective planets.

* ''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 an [=EpiPen=], and then [[LampshadeHanging lampshades]] how surprising it is that it worked.

** 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 they 'feed' (having "given up their digestive systems") by injecting themselves with human blood. Oh, how science has marched on.

* Discussed in ''Machinima/FreemansMind''. Freeman wonders if several of the things that he encounters carry alien pathogens, and hopes that alien DNA is different enough to not affect him.

* ''Literature/SpeakerForTheDead'': The humans colonists on Lusitania and their crops are infected by a native virus, the Descolada. [[spoiler:The Descolada was engineered to specifically be able to [[LegoGenetics adapt to different genetic codes]], and the virus may be semi-intelligent itself.]]

* 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:But he was the one behind the poisonings, and the protagonist realized he had been telling a deliberate lie.]]
* In the ''Literature/TheExpanse'' novel ''Cibola Burn'', there's no actual alien disease on Ilus. Instead, what happens is [[spoiler: one of the local lifeforms, an algae-analogue that lives in the planet's clouds, turns out to be very well suited to the eye's vitreous body. The immune system isn't very good at fighting it off, so it steadily renders everyone blind. For some reason, it's vulnerable to oncolytic drugs.]]

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

* The absence of such barriers is an important plot point in ''LightNovel/InvadersOfTheRokujyouma''. When Koutarou is transported to Forthorthe 2000 years in the past, he's exposed to a virus that debilitates the Forthortheans but only causes minor symptoms in him and Clan (a modern Forthorthean). This is not because the former is human, but because the two of them have genes for resistance to the virus, which Clan uses to develop gene therapy to treat the virus. The odds of Earthlings and Forthortheans having genomes similar enough for this to happen is lampshaded in-universe, with speculation that the two species are one and the same and were deliberately placed on their respective planets.
* ''Literature/MassEffectAnnihilation'': Pointed out early on in the plot by one of the characters. There's a disease spreading through several species. Except their biologies are so different (such as the ammonia-breathing volus) this should be ''impossible''. It's what clues the characters in that the disease is artificial.

* A rare [[JustifiedTrope Justified]] (and partially [[AvertedTrope Averted]]) scientifically plausible example appears in the ''Literature/RiftersTrilogy'' written by Creator/PeterWatts, a former marine biologist and hard science fiction writer who [[ShownTheirWork shows his work]] to the point that at the end of his books he includes a section explaining its science complete with references and citations of actual academic papers. In it almost all life on Earth originated from Mars via {{Panspermia}}, however life had already developed on Earth near its hydrothermal vents where they stayed for billions of years while terrestrial life dominated the planet. This tiny nanobacterium, later dubbed βehemoth, evolved to be extremely efficient to eek out life on the ocean floor as its biology meant that both high salinity[[note]]the amount of salt in water[[/note]] and low temperatures interfered with its metabolism. Then a geothermal energy project accidentally carries it out of its native habitat and it turns out that its extreme efficiency means it absolutely thrives in and can outcompete any terrestrial organism in any environment which isn't as cold and salty as the sea floor, a description that includes virtually all of Earth ''including the inside of living cells'' in which it replicates using the abundant amount of nutrients present until the host runs out and dies. The partial aversion comes in that while βehemoth has a close enough biochemistry to normal life that it can eat it βehemoth has evolved such a mineralized cell wall that normal life can't even recognize it as alive, meaning there's no immune response and any organism that could eat βehemoth wouldn't even recognize it as food. The end result? ThePlague, [[AlienKudzu Technically-Not-Alien Kudzu]], and a [[ApocalypseHow/Class5 Class 5 Apocalypse How]] in the making all wrapped into one.

* In the ''Literature/SpeakerForTheDead'' trilogy the humans and their crops are infected by a virus, the Descolada, on the alien planet. [[spoiler:The Descolada was engineered to specifically be able to adapt to different genetic codes and the virus may be semi-intelligent itself]].

* The ''Literature/VorkosiganSaga'' again: the notorious and disgusting worm plague of Sergyar. "It wasn't all that lethal, as plagues go."
* In Creator/HGWells' original ''Literature/TheWarOfTheWorlds'' the Martians are killed off by a terrestrial microbe within several weeks of their arrival. This is almost a reversal of the trope, as it's rather explicitly stated that the aliens are ''more'' vulnerable to Earth microbes than humans, due to not having evolved and adapted alongside them; this is based upon the outbreaks that can occur when two previously isolated human cultures begin interacting and exchanging diseases, unintentionally or otherwise. This is because ScienceMarchesOn; the very idea of viral transmittance was new when the book was first written.
** ''Literature/TheWarOfTheWorlds'' 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.
** It's also mentioned that obviously, Earth's gravity is higher than on Mars, and this slightly affects them: Wells's Martians are basically blobby heads with many tentacles attached (looking sort of like an octopus). They evolved alongside their advanced technology to the point that all of their other organs atrophied except for the brain and "hands" (which turned into tentacles). The narrator mentions that it is believed that on Mars, the Martians actually walked around on their tentacles, like spiders. In Earth's higher gravity, however, it uses up all of their strength just to push themselves around. The problem, of course, is that they don't need to be able to move much to pilot their robotic vehicles, which are militarily far superior to anything the humans can throw at them.
** Of course, the book was intended as an unsubtle critique of colonialism, in a time period where Britain was conquering the tropics and their troops were falling victim to malaria and other diseases they had no resistance to.
* ConversationalTroping in the ''Literature/DoctorWhoNewAdventures'' novel ''Death and Diplomacy'', when Benny worries that she may have caught an alien STD from Jason. Jason points out that the chances of a compatible disease are pretty remote, and Benny retorts that that if it's possible for one to exist ''at all'', then [[ExtremeOmnisexual Jason probably has it]].
* The ''Literature/XWingSeries'' had the Krytos virus, a deadly and highly contagious disease that rapidly jumped species. Justified by the fact that Krytos was artificially engineered to be able to jump species, and consisted of multiple strains that targeted totally different racial groups so that each could jump to other, similar races.