No Biochemical Barriers / Cross-Species Disease

Examples of No Biochemical Barriers involving diseases.

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Anime & Manga

• In Dragon Ball 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 nutrition and reproduction are concerned.
• The V-Type Infection from Macross Frontier, which comes from bacteria that live in Vajra, and is lethal to humans and Zentraedi (though it takes a few years to actually kill someone). Ranka Lee is the only known person to have been infected and survived; indeed, she's completely immune to it, speculated to be because she contracted the disease when still in her mother's womb. It turns out that the bacteria are the mechanism that allows the Vajra's Hive Mind, and the human/Zentraedi infections were the Vajra's attempt to communicate with them. The problem is that in most infections, the bacteria went for the host's brain, and caused lethal damage. The bacteria in Ranka settled in her intestines, where they achieved symbiosis. Once the Vajra figure this out at the end of the series, it is implied they are able to influence the bacteria in other infections to do the same, preventing any further deaths.
• Mobile Suit Crossbone Gundam: Ghost has an alien microbe that the villains plan on releasing on Earth, which the heros are trying to prevent. The logic is that since the microbe has a completely alien biochemistry, nothing on Earth will be able to defend against it, and it will spread like wildfire and wipe out any life it comes across. This ignores the fact that the opposite should be equally as true: the microbe should be just as defenseless against Earth's microorganisms. Or, in the most likely case, absolutely nothing happens, and the alien microbe dies due to lack of its native environment.
• Space☆Dandy: Episode 4 has a zombie virus that infects every single species in the galaxy, including humans, all kinds of aliens and robots.
• In one of episode of Nyaruko: Crawling with Love!, the Humanoid Alien 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 Weaknesses. 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.

Comic Books

• The Green Lantern Arc Sinestro Corps War has a virus called Despotellis capable of killing any species, even ones normally resistant to disease. This being a superhero universe, Despotellis is sentient.
• Played reasonably straight in Alan Moore's Top 10 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 non-viable lifeform (and in one case a god). Interestingly it is said to be the result of a city full of humans, super human, mutants, aliens, robots, magical creatures and all the variations from alternate universes all interacting sexually with each other.

Fan Works

• Everfree Infection AU: The version of the Everfree infection that's been devastating mammals is shown to infect and produce near-identical symptoms in changelings, which are basically large pony-shaped insects. Making things more confusing, said infection originally only affected bugs and made the jump to non-arthropod life in the first place due to explicitly magical interference.
• "Solaere ssiun Hnaifv'daenn": Because bloodfire already managed to jump species once before (in the TNG novel Death in Winter, from Kevratans to Romulans), Doctor Emira t'Vraehn gives the human Jaleh Khoroushi a dose of the vaccine, just to make sure the disease doesn't develop a taste for humans next.

Films — Live-Action

• The Facehuggers in the Alien films are somehow able to impregnate species (like humans and dogs) that would not have been encountered in the environment in which they evolved or by the species that engineered them (the backstory is unclear on which is the case) as long as it is warm-blooded and alive. This is unsatisfactorily explained by claiming that it is capable of adapting itself to the DNA of the host and incorporate features of said host into the Chestburster it produces, but this does not account for how it is able to do so with a completely unknown species with a vastly different biology. If we suppose it was engineered rather than evolved, it seems likely that its creators would have designed it to target a specific species, because making it so adaptable that it could infest completely unknown species would surely mean that it could adapt to infest the species that created them. (Which, according to some sources, is exactly what happened...) As far as in-movie canon goes, Alien: Resurrection makes it clear that the Alien and host's DNA are somehow mingled, even in the host's own blood. That's still crazy from a biochemistry point-of-view (why would aliens even have DNA?), but at least they try to explain it.
• In Day of the Animals, practically the last line in the movie informs us that the animals all went berserk because of a "virus". This list includes wolves, dogs, birds of prey, bears, cougars, rattlesnakes, tarantulas, and a New York Ad Exec.
• Doctor Strange in the Multiverse of Madness: A cross-dimensional example is discussed but otherwise ignored. The Illuminati place Strange and America in polycarbonate glass chambers to ensure that they don't spread any alt-universe diseases that their own universe wouldn't have any exposure to or a cure for. This seemingly isn't a problem however, since America has been dimension-hopping for years now, apparently to at least 72 worlds, and she isn't carrying anything.
• The alien organism depicted in The Thing (1982), and its 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 up to eleven.

Literature

• In The Andromeda Strain, the alien "virus" (actually, a weird crystalline lifeform) 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, but from his jet crashing after an intermediate form of the virus consumes all the plastic seals in his jet.
• Animorphs:
  • The premise of the series is sentient parasites known as the Yeerks are invading Earth and taking over the bodies of humans, as they had with other races such as the Hork-Bajir, the Taxxons, and a single Andalite. While the evolution history of the Yeerks is unknown (especially their relation to their more benevolent counterparts, the Yoorts), they had evolved on their homeworld using the simple and primitive Gedds as their semi-symbiotic hosts. However, they still manage to gain control of other species, even though they have different brain structures and biological instincts.
  • In one of the books, 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 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 Arthur C. Clarke's Before Eden, astronauts discover lithovore slime-like life on Venus... only to wipe it out by inadvertently introducing pathogenic microorganisms from Earth.
• David Starr, Space Ranger: A series of poisonings occurs in people who ate Mars-grown food. A (human) Martian scientist says that it could have been caused by the local bacteria. He's the one behind the poisonings, and the protagonist realizes that he's been telling a deliberate lie.
• Discussed in the Doctor Who New Adventures 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 Jason probably has it.
• The Expanse: In Cibola Burn, there's no actual alien disease on Ilus. Instead, what happens is that 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.
• 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 astronaut's 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 the Hainish Cycle by Ursula K. Le Guin; measles is described as a cross-species disease. In The Dispossessed; 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 Human Aliens 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.
• Honor Harrington:
  • 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 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.
• The absence of such barriers is an important plot point in INVADERS of the ROKUJYOUMA!?. 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, with speculation that the two species are one and the same and were deliberately placed on their respective planets.
• This is pointed out early on in Mass Effect: Annihilation by one of the characters. There's a disease spreading through several species, but their biologies are so different (such as the ammonia-breathing volus) that this should be impossible. It's what clues the characters in that the disease is artificial.
• Rifters Trilogy: A rare justified (and partially averted), scientifically plausible example, thanks to author Peter Watts being a former marine biologist who 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 this setting, almost all life on Earth originated from Mars via Panspermia — however, life had already developed on Earth near its hydrothermal vents, where it stayed for billions of years while terrestrial life dominated the planet. This tiny nanobacterium, later dubbed βehemoth, evolved to be extremely efficient to eke out life on the ocean floor, as its biology meant that both high salinity^note the amount of salt in water and low temperatures interfered with its metabolism. When a geothermal energy project accidentally carries βehemoth out of its native habitat, it turns out that its extreme efficiency allows it to absolutely thrive and 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 is that while βehemoth's biochemistry is close enough to normal life that it can eat the latter, βehemoth has evolved such a mineralized cell wall that normal life can't even recognize it as alive, so there's no immune response and any organism that could eat βehemoth wouldn't even recognize it as food. The end result: The Plague, technically-not-Alien Kudzu, and a Class 5 Apocalypse How in the making all wrapped into one.
• Speaker for the Dead: The human colonists on Lusitania and their crops are infected by a native virus, the Descolada, which was engineered to specifically be able to adapt to different genetic codes and may be semi-intelligent itself.
• Star Trek Expanded Universe:
  • One series deals with a plot to infect all Alpha Quadrant species with 100%-mortality diseases. The first book in the series presents a virus that specifically infects characters of mixed heritage.
  • In Death in Winter, the first book in the Star Trek: The Next Generation Relaunch, bloodfire manages to jump species from Kevratans to Romulans.
• The notorious and disgusting worm plague of Sergyar from the Vorkosigan Saga. "It wasn't all that lethal, as plagues go."
• The War of the Worlds (1898):
  • 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 Science Marches On; the very idea of viral transmittance was new when the book was first written. Of course, the book was intended as an unsubtle critique of colonialism, in a time period when Britain was conquering the tropics and their troops were falling victim to malaria and other diseases which they had no resistance to.
  • 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.
  • 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.
• The X-Wing Series has the Krytos virus, a deadly and highly contagious disease that rapidly jumps species. Justified by the fact that Krytos is artificially engineered to consist of multiple strains that target totally different species groups so that each can jump to other, similar species.

Live-Action TV

• In the Agents of S.H.I.E.L.D. episode "FZZT", a Chitauri helmet infects some firemen 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.
• Babylon 5 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. Other aliens also turn out to be immune—for example Lennier and Delenn (Minbari) spent hours in isolation with the Markab but were unaffected by the plague. There were also no reports of the plague jumping to other species. 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 - too late to save the Markab from apparent extinction.
• In Battlestar Galactica (1978), 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...
• Doctor Who:
  • "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.
  • "Orphan 55": Hopper viruses are multiplatform diseases that can infect both organic beings and technological devices. The Doctor implies they can infect anything complex enough.
  • "Praxeus": Played With. Praxeus is equally deadly to both Earth lifeforms and Suki's species, but the cure the Doctor develops is specific to humans. When Suki tries to use the cure on herself, she's killed from an advanced Praxeus infection.
• Mighty Morphin' Power Rangers: In the opening three-parter of the third season, the Repellator is briefly afflicted with Kimberly's cold after she sneezes on him a couple times. Lord Zedd's reaction is hilarious.
• Space: Above and Beyond. Having captured an alien prisoner, the protagonists wonder what to feed it. They decide that water is safe enough. The alien promptly dies (though it may have committed suicide rather than remain a captive).
• Stargate SG-1:
  • In one episode, the local alien population of a planet are falling ill after the arrival of the main team. As the good guys desperately try to find a cure, General Hammond points out that they're lucky they mostly deal with human civilizations who have the same diseases as Earth do and that's it's a small miracle they haven't run into a problem like this before. The episode was ultimately an aversion, since it turned out that the illness wasn't caused by a disease.
  • 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... 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.
• Star Trek:
  • A popular plot device, possibly because, as per TNG: "The Chase", most if not all of the galaxy's humanoid species share a common ancestry. It's still weird given the physiological differences (take the Vulcan/Romulan species, which has copper-based blood).
  • 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 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.
    • 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 disease itself but thinks he carries the disease so would be stuck on the planet in a self imposed quarantine unless a cure is found.
  • In Star Trek: Voyager the Phage, the disease afflicting the Vidiians, is quite capable of jumping the species barrier (to the point where replacing the afflicted organs with ones harvested from non-Vidiians only slows it down), though Klingon immune systems can evidently fight it off.
• Tracker (2001) 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 Mate or Die 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 Fridge Horror given how close he came to infecting Mel, who was later revealed to be a Half-Human Hybrid. She might have died faster.

Tabletop Games

• Warhammer 40,000:
  • The Life-Eater virus can scour entire planets clear of life in minutes, no matter what kind of life inhabits said planet. Even Eldar (with 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 nano weapon.
  • Nurgle's Rot is another disease that can effect all life. This one is a bit more justified because the disease is created by the god of plague, and is more of a 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, it corrupts the souls of its victims into Plaguebearers.

Video Games

• Purposely averted in The Dig, 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. This doesn't stop the characters from falling prey to a disease of a different sort later on.
• In The Elder Scrolls, 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 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 every surviving race on the continent putting aside their differences to attack the Sload in retaliation.
• Halo:
  • The Flood can infest any creature with enough biomass and individual sapience to support the parasite. This specifically excludes Hunters and Drones (being The Worm That Walks 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 they can't infect Hunters), and has plenty of calcium (which is why they can't infect Drones).
  • Lampshaded in Halo: Broken Circle, where an Elite on a mission to the Prophet homeworld notes he and his team been treated with special antibiotic, microscopic nanoagents meant to destroy the local antigens invading their system before any harm can be done. Presumably, all members of the Covenant have similar treatments done at an early age in order to function on various foreign worlds.
• I Was a Teenage Exocolonist: Curing the Shimmer becomes a pressing issue precisely because it manages to jump to humans despite having evolved to affect Vertumna's wildlife.
• Played with in Mass Effect:
  • Used as a plot point in Mordin's recruitment mission: everyone you speak to in the plague zone knows the disease ravaging the area has to be an intentionally released bioweapon precisely because it's infecting every species except humans (and vorcha, but they're immune to everything).
  • 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 (a non-sapient quadruped).
    Mordin: Implications... unpleasant.
  • A minor plot point in Mass Effect: Andromeda: one human colonist that's woken up is found to have a deadly and extremely contagious disease. She runs from the Nexus before it becomes contagious, but then everyone worries that if the angara (the natives of this sector of Andromeda) can contract it, there could be a pandemic. In the end, the woman is located and it's discovered that the angara are totally immune to her disease... so some alien-hating angara hit upon using the disease as a bioweapon against the Milky Way "invaders".
• Played straight in Space Empires. Once researched, Plague Bombs and other forms of bio-weaponry will be equally effective against all races.
• Star Control Origins:
  • 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 tries to avoid infecting people if at all possible.
  • The Gloosh's original home world was wiped out by a disease accidentally brought to their planet by a group of interstellar explorers. The explorers were deeply remorseful and helped the Gloosh move to a new homeworld... while being more careful about cross-contamination this time.
• The Zerg Hyperevolutionary Virus from 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 Psychic Powers supercharging their immune systems.
• Subnautica has an alien pathogen that specifically alters the infectee's biochemistry to be more compatible with itself. Presumably, the rapid mutation is what actually kills the infectee.
• In Sword of the Stars, the disease bio-weapons need to be invented only once and are equally useful against all species. They can be fired in a first-contact situation (before you'd logically be able to dissect a member of said species and find their disease markers) and will not lose any efficiency. The only exception is the Zuul, who are immune to all plague weapons except for the Grey Goo missile no matter what you try.
• Your Mission Control in System Shock 2 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. This turns out to be a subversion, though, as the life found there is ultimately of terrestial origin — and your Mission Control is responsible for turning into what it has become..
• The Turing Test: Apparently, a virus that normally infects an organism found only in Europa is also able to infect organisms from Earth, despite the species selectiveness of real-life virus.
• In Warcraft III and World of Warcraft, although the Plague of Undeath created by the Lich King only affects humans and not any of the other intelligent species in the game universe, it still has mutagenic effects on plant and animal life. Probably because it's magic and was made to prepare the land for undead invasion. In at least one quest it's explicitly stated to affect some other races, but not fully as intended; one quest involves getting bones from Murlocs that were turned into undead by the plague... but weren't robbed of their free will by it as humans would be.
  • Warcraft also has Half Human Hybrids, despite diseases apparently not translating well among species on the same planet. For example, Garona Halforcen who is half orc and half DRAENEI. Yup, alien parents from different planets. And then she had a son with MEDIVH, who is, biologically, human. So that's 3 species from 3 different planets who are all compatible with one another. Draenei and Orcs in particular are interesting, since their hybrids are clearly still fertile.
  • When the Lich King first came to Northrend, he had difficulty fighting the Nerubians because they couldn't be infected and killed directly by the plague, but they weren't immune to the Scourge's Necromancy, so once some of them got killed by non disease means, they were able to be resurrected as undead slaves. This was later applied to other races as well, which is how you get both the Death Knight class (which can be any playable race except Pandaren) and Dranosh Saurfang.
  • Most humanoid races are affected by the undead plague. Humans, elves, dwarves, gnomes, orcs, and draenei (both from other worlds) can definitely be affected. Humans afflicted with the worgen curse are not affected, however like the Nerubians above, they can be killed by traditional means and then raised via necromancy. The plauge is magical in nature though, so A Wizard Did It.

Webcomics

• Awful Hospital: Exaggerated with the villains' Assimilation Plot, a malady that can spread to absolutely anything in The Multiverse, regardless of biology or infection vectors.
  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."
  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 Kevin & Kell, a virus that appears to be Covid-19 in all but name is shown as something all the Funny Animal characters have to take precautions against. In real life, there is still debate as to which animals can even be carriers, never mind sick. The science is pretty clear that reptiles and amphibians, such as Greta the garter snake and Todd the toad, shouldn't have anything to worry about at all.

Web Original

• Discussed in Freeman's Mind. 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.
• SCP Foundation: 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 lampshades how surprising it is that it worked.

Western Animation

• American Dad!: Lampshaded (but otherwise played straight) with Roger as early as the second episode: he claims he’ll be able to survive a deadly viral outbreak by virtue of the fact that he’s not human and couldn’t catch something like that. Stan immediately contradicts him by pointing out the cold sore on his lip, to which Roger has no good explanation.
• In one episode of Ben 10, 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 fire to ice. Played for Laughs in Ben 10: Alien Force when Vilgax ends up catching a cold from Ben.
• Played for Laughs in a Sick Episode of Harry and His Bucket Full of Dinosaurs, where not only do the dinosaurs catch the cold from Harry (a mammal), they each get a different disease specific to their own species.
• Justice League: One episode had a rogue Amazon 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.
• While The Owl House averts this when it comes to food, Luz ends up getting sick with the common mold in the episode "Eclipse Lake". Amity even expresses concern over this, since -while it's normally harmless- they have no way of knowing how badly it would affect a human.
• In one episode of Star Trek: The Animated Series, 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.
• Star Wars: The Clone Wars: "Blue Shadow Virus"/"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.

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.
  • Some bacteria like C. botulinum can kill very wide ranges of animals. But this is actually an aversion, because it is the toxin produced by the bacteria that is dangerous rather than a bacterial disease.
• 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 carnivores such as ferrets and cats.
  • 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.
• 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.
• 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), but it was possible because the man was also 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).
• Even stranger are Bunyaviruses, Reoviruses and Rhabdoviruses, which can infect both animals and plants.
• Leprosy, a bacterial disease that mainly infects humans, strangely can also be carried on the skin of nine-banded armadillos.
• 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. However, 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.
• Toxoplasmosis is an interesting Zig-Zagged example: this hearty parasite infects most warm-blooded animals, found world-wide in everything from birds to whales to humans. In most species, infection causes flu-like symptoms and mild-to-moderate neurochemical disruptions, and can be chronic until treated but is rarely life-threatening. In rodents, however, it causes distinct and extreme changes that suppress fear and danger-aversion while inducing literal Adrenaline Junkie addiction responses to reckless exploration- all working to encourage spread via feline predators, in whom the parasite is particularly mild. In Australian Marsupials, meanwhile, infection is frequently and rapidly lethal as the parasite evolved and spread after their isolation- while the parasite is versatile enough to adapt to Australian Marsupials, Australian Marsupials' immune systems have no adaptations to handle Toxoplasmosis.

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