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My world - basically Earth all along, unless told otherwise - is populated by humans and few fantasy-based "races" (actually, species - any interbreed offspring, if possible, is infertile): elves, dwarves, etc. Despite the fact that setting is to some extent inspired by common fantasy tropes, there is no magic at all, just hard science. All "fantasy species" in fact belong to the same Homo genus and should be plausibly justified as the effects of natural evolution of (perhaps isolated) groups of hominids.

All of these species keep at least some contact with each other, sometimes even living together in one city.

How likely is that somebody will get infected with a disease from a member of another "race"? I know that most of the diseases are rather species-specific, but there are some exceptions (afaik HIV originates from apes).

Will halflings be just ok when the plague is decimating human communities? Can a dwarf get infected by human HIV or syphilis?

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    $\begingroup$ If everybody belongs to the same Homo genus, diseases are likely to be common. $\endgroup$ – Alexander Oct 10 '17 at 19:50
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    $\begingroup$ Let's not forget about parasites: they can live in multiple hosts for different life cycles, and could be a cause of common, inter-species diseases. Also +1 for making the leap and calling them fantasy species- the "race" thing always bugged me. $\endgroup$ – PipperChip Oct 10 '17 at 20:47
  • $\begingroup$ Might be easier to see this for disease of other animals as there are multiple species in the genus of other animals but only 1 in homo. AFAIK there are a lot of diseases that can cross to related species and a lot that don't. $\endgroup$ – A. C. A. C. Oct 10 '17 at 21:21
  • $\begingroup$ Great apes like chips can catch human colds, so presumably your elves and dwarves can as well, unless magically protected. But it's also likely that something will be more severe in one of your humanoid varieties than another. $\endgroup$ – Chris Stratton Oct 11 '17 at 5:48
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First, disease from bacteria and viruses do tend to be species-specific; however, zoonosis can occur. The chance that this can happen isn't really a number but more related to frequency of exposure instances where said pathogen interacts with another species. The root principal is the more chances you give a pathogen to evolve, the more likely it will.

The theory behind AIDS is that it arose from the predation of primates. Because we frequently ate them we gave the virus more chances to adapt to our physiology.

Bird flu is another example. Pigeons and crows liter our settlements and we are constantly exposed to their fecal matter. That constant exposure gives it a chance to adapt and then spread.

Another aspect is carriers and survivability of the pathogen. In some species a pathogen can have no effect yet still be communicable to other species. Also the structure of it matters too. If a pathogen can survive exposed to air it can be carried by other species and left on surfaces you interact with.

So could a species suddenly get a human pathogen? Not impossible, but unlikely.

Could dwarves get syphilis? Depending on their genetic divergence from humans and the amount of exposure from them, maybe, maybe not.

The chances are like winning a lotto where each interaction is a ticket.

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    $\begingroup$ That's one kind of lotto I'd avoid for life $\endgroup$ – Vylix Oct 12 '17 at 1:39
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There are many bacteria and viruses that can infect several species, possibly with different symptoms or prognosis.

An example is cow pox which is not very dangerous for humans, but they can catch it.

Quite the reverse with avian flu.

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  • $\begingroup$ And cowpox is close enough to smallpox to grant resistances so it was a net win for milkmaids. $\endgroup$ – user25818 Oct 10 '17 at 19:55
  • $\begingroup$ @notstoreboughtdirt: exactly. It was also the breakthrough that lead to vaccines (vaccines radix is from "vache" , "cow" in French). I just wanted to point out many diseases are already inter-species, even with species father away than "hominids" genus. All various combinations can be exploited in the plot, depending on the need. $\endgroup$ – ZioByte Oct 10 '17 at 20:15
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Do you mean how likely it is that someone will get infected from a member of another species? If so I would say the chances are very high that some diseases would eventually cross the species barrier. Generally the closer a species is related to another and the closer the contact between them the more likely transmission of disease is. That said it doesn’t always happen, it’s just more likely.

The original source of human HIV was thought to have been monkeys and SARS came from chickens. (so they don’t have to be that close!). The details of what is or isn’t transmitted depend very much on the specific biochemistry involved and all cases are possible. So one species might be infected with another species disease and be affected the same way, affected to a lesser extent or not affected at all.

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Your best bet would be Prion diseases.

Prions aren't viruses or bacteria, they are folded proteins. Which just so happen to mess with protein expression in the organisms they affect.

The infamous Mad Cow disease, more precisely known as Bovine Spongiform Encephalopathy, is an example of prion disease.

Prion diseases are very likely to achieve zoonosis. And since your races are all mammals with a central nervous system, they could very likely wreak havoc.

As an extra, prion diseases have been in some instances linked to cannibalism. So you can have a gruesome cannibal cult being the cause of the terrible disease that afflicts your world.

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    $\begingroup$ Welcome to WorldBuilding Pablo! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Oct 11 '17 at 19:28
  • $\begingroup$ I believe the OP's desire is to limit zoonosis, a prion would be worse. $\endgroup$ – anon Oct 11 '17 at 19:30
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I would read up on zoonoses! These are infectious diseases that, for a host of reasons, are able to make the leap from one species to another. Many of humankind's most terrible diseases are zoonoses: Ebola, flu, various poxes, plague, Zika, tuberculosis, many worms/parasites.

Zoonoses usually have a reservoir host species, in which the infectious organism causes little or no harm (avian influenza causes no illness in wild aquatic birds but if it makes the leap to us, can be deadly). The organism exists happily in this host, duplicating itself and carrying on until chance exposures lead to crossover events into a brand new species.

The Ebola outbreak in West Africa was hypothesized to have begun when a group of children played near a tree that was filled with Ebola-carrying bats and became infected. From the Ebola virus' perspective, it's not particularly useful to jump over to a human host that you are just going to kill off immediately- this does not promote the survival of the species! It was just a chance encounter that led to crossover- a matter of proximity and exposure.

Also, over time, killer viruses tend to become less potent in new hosts (HIV and some Ebola strains have done this)- this is a way of adapting and ensuring your host survives long enough to pass virus to lots of people. This evolution occurs once the organism has made the leap into humans and begins to adapt to the human immune system.

A cool example of virus evolution example is influenza, which has a variety of surface proteins that help the virus adhere to different mammalian species' cells with varying specificity. These proteins' genetic code "drift" and change over time naturally, leading to different strains of virus such as H1N1 or H3N2. If it ever shifts to a code that produces surface proteins that fit human respiratory cells receptors exactly (think like a lock and key), then we're looking at a pandemic event. Genetic change can happen faster when different virus strains recombine, like how swine flu was a mix of genes from human, avian, and pig-specific virus. Other animals like dogs are not susceptible to catching flu at all, because their receptors look totally different from ours/pigs/birds, which are all close enough to be shared around once in a while.

If you look up the life cycles of parasites like malaria or schistosomes, they often have complex relationships between different species in order to survive. Malaria has to, at different stages of its alien-esque life cycle, live in mosquitoes, humans, and the environment. Parasites can infect a ton of different species, which may or may not be the primary host, dead-end host, or reservoirs.

Anyway, you could certainly have a disease that halflings are immune to, or are reservoir hosts of, or can be infected by but only experience milder symptoms.

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