History NoBiochemicalBarriers / CrossSpeciesDisease

* 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 alien organism depicted in ''Film/TheThing1982'', and its [[Film/TheThing2011 prequel]], is a single-celled life form, capable of invading the body of any terrestrial animal (including humans), and taking over that animal cell by cell, until it has perfectly imitated the animal. This includes all knowledge that animal had, allowing an imitated person to perfectly pass as that person indefinitely, as well as pass as any animal, integrating seamlessly into, say, a group of dogs. This concept takes this trope UpToEleven.

* ''Webcomic/AwfulHospital'': Taken UpToEleven with the villains' AssimilationPlot, a malady that can spread to absolutely anything in TheMultiverse, regardless of biology or infection vectors.

* One of the most unusual examples of this trope in real life occurred to a Colombian man diagnosed with cancer in 2014. Extraordinarily, it was found the cancer cells were from a ''tapeworm'' which the man was ''also'' infected with. Normally it should be impossible for cross-species cancer infection (heck, contagious cancer as a whole is quite uncommon, and otherwise ''only'' occurs within the same species), [[JustifiedTrope but it was possible]] because the man was ''[[UpToEleven also]]'' [[ContrivedCoincidence infected with HIV]], and a compromised immune system couldn't stop the tapeworm's tumour from migrating to the man's own tissues (and who, unsurprisingly, died shortly after he was diagnosed).

* The Severe acute respiratory syndrome coronavirus 2 ([=SARS-CoV-2=] or COVID-19) first appeared in humans who visited a certain live animal food market, and though the exact origin is currently unknown, it seems to have originated from a bat coronavirus and likely passed through at least one other animal species before infecting humans. The closest known relative is a virus found in pangolins, and it is capable of infecting ferrets and feline species like domestic cats and tigers.

* In one ''Literature/{{Animorphs}}'' book, Ax catches a disease. All of the human Animorphs but Cassie come down with apparently the same disease, which manifests as the flu in humans (though it's much more potentially ''fatal'' to Ax.) Worse, the disease seems to affect everyone ''even in morph'', including [[ShapeshifterModeLock Tobias]], who spends very little time in human form. Having morphing powers seems to be what made them susceptible: no other humans caught it.

* In one episode of ''WesternAnimation/Ben10'', Ben comes down with a (human) cold. This persists in each of his alien forms, but with different symptoms; Heatblast, for example, gets his powers reversed from [[PlayingWithFire fire]] to [[AnIcePerson ice]].

* 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 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 reproduces 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.

** Also used humorously in a throwaway line by Mordin on the ''Normandy'', where he mentions he's trying to figure out how a "Scale Itch" infection got on-board... considering it's an STD carried only by varren.

* In one of episode of ''LightNovel/HaiyoreNyarkoSan'', 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 absence of such barriers is an important plot point in ''LightNovel/RokujyoumaNoShinryakusha''. 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.

** 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. They can infect just about any lifeform that is carbon based, has a large enough central nervous system (which why the can't infect Hunters), and has plenty calcium (which is why they can't infect Drones).

* ''Series/BabylonFive'' had the Markab people fall victim to a universally fatal plague to which humans turned out to be completely immune, as it acted on a type of nerve cell that humans don't have. The Pak'ma'ra, however, turned out to be similar enough that it also affected them, though less severely. In fact, that similarity allowed Dr Franklin to figure out how to cure the disease - [[spoiler: too late to save the Markab from apparent extinction]].

* ''Literature/MassEffectAndromeda'': 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.

* Invoked 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]].

-->'''Crash:''' IT SHOULDN'T EVEN BE POSSIBLE FOR THE SAME ILLNESS TO ACHIEVE CROSS-CONCEPTUAL-CONTAMINATION THIS ABSOLUTE ... IMAGINE IF YOU COULD "CATCH" AN AGGRESSIVE CANCER AS EASILY AS YOUR "COMMON COLDS."\\

-->'''Crash:''' IT SHOULDN'T EVEN BE POSSIBLE FOR THE SAME ILLNESS TO ACHIEVE CROSS-CONCEPTUAL-CONTAMINATION THIS ABSOLUTE ... BUT, TO DESCRIBE IT IN WORDS YOU MIGHT FIND SUITABLY ALARMING, IMAGINE IF YOU COULD "CATCH" AN AGGRESSIVE CANCER AS EASILY AS YOUR "COMMON COLDS."\\
GOT THAT? GOOD, NOW IMAGINE IF YOU COULD CATCH IT JUST AS EASILY FROM YOUR NEIGHBORS, A PINEAPPLE, A PICNIC TABLE OR THE SOUND OF A TROMBONE. THAT'S WHAT WE'RE DEALING WITH HERE. AT LEAST, THAT'S WHAT THE PARLIAMENT'S SHOOTING FOR. THE OMNIVIRUS. THE ALPHA MALADY.

* In ''WesternAnimation/Ben10'', 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]].

* The Severe acute respiratory syndrome coronavirus 2 ( SARS-CoV-2 or COVID-19) first appeared in humans who visited a certain live animal food market, and though the exact origin is currently unknown, it seems to have originated from a bat coronavirus and likely passed through at least one other animal species before infecting humans. The closest known relative is a virus found in pangolins, and it is capable of infecting ferrets and feline species like domestic cats and tigers.

* The Wuhan Coronavirus (or COVID-19) first appeared in humans who visited a certain live animal food market, and though the exact origin is currently unknown, it seems to have originated from a bat coronavirus and likely passed through at least one other animal species before infecting humans. The closest known relative is a virus found in pangolins, and it is capable of infecting ferrets and feline species like domestic cats and tigers.

* 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 (or COVID-19) first appeared in humans who visited a certain live animal food market, and is believed to be a bat coronavirus that changed as a result of passing though one or more other animal species at that market.