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Following on from a recent question about animals, concerning a group of people traveling sideways in time, to a world that resembles Earth as it would have been had humans never evolved, it occurs to me that in the tropics, diseases and parasites are far more dangerous than big animals.

Mosquitoes, as far as I know, target all warm-blooded vertebrates, so if you visit a world where humans never evolved, they will still go after you. But what about the diseases they transmit, like malaria and yellow fever? Are those shared with other species, so they will still get you? Or are they species-specific, so that (at least initially, until they make the jump from some animal) you don't need to worry about them?

Same question about tsetse flies and sleeping sickness. I think the answer for that one is definitely bad news, sleeping sickness hits humans and animals?

Same question about the various nasty parasitic worms such as the Guinea worm. Are they adapted to humans specifically, or would they be automatically able to target humans by virtue of being warm-blooded vertebrates?

(For diseases of the temperate zones, I know some of the answers already. Flu, measles, tuberculosis, smallpox are all good news; it will take a long time for them to jump from animal hosts. Rabies is bad news, as are some parasitic worms transmitted by dogs. And bubonic plague, transmitted by fleas from rats. I think Lyme disease is also bad news.)

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    $\begingroup$ All u mention is less species specific than one would like it to be, so there always should be a degree of concern. Some stuff sticks, most may not, but the few which stick may be enough. For insects, worms all mammals are the same, and not only mammals. Flies feast on anything. This direction of problems u hit a jackpot $\endgroup$
    – MolbOrg
    Mar 26 at 9:16
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    $\begingroup$ The bacterium which produces human tuberculosis, Mycobacterium tuberculosis, grows only in humans. Unfortunately, its cousin Mycobacterium bovis, which produces bovine tuberculosis, is very much less selective and is happy to infect cattle, humans, pigs, felines and canids (including foxes and wolves, both wild and domesticated). Killing it was one of the main original goals of milk pasteurization. $\endgroup$
    – AlexP
    Mar 26 at 12:40
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It's very species dependent. In general, viruses have a hard time jumping the species barrier unless you live in areas where large numbers of people are in contact with large numbers of animals and their bodily products. That's why so many infectious diseases (swine flu, avian flu, SARS) come from Asia, where you have large numbers of people in contact with large numbers of poultry and hoofstock with relatively lower hygenic conditions (especially in the more rural areas where infrastructure is still developing) than most Westerners are used to. Similarly, the 1918 flu is thought to have come from pig farms in Kansas.

However, this isn't the only way viruses can jump species, as Ebola and probably COVID can tell you. The thing is whether a virus can jump from species to species is a very scattershot process. Both Ebola and COVID only jumped species because there is a thriving bushmeat market in both regions, and hence there were so many interactions between humans and wildlife the prospect that one chance encounter would spread the mutated disease is more likely.

I would be more worried about parasites. Studies have found that in pre-industrial societies where the population is too spread out for viruses to be a big deal parasites are typically a bigger detriment to individual health than viruses are. And parasites have been known to jump species, many of our current tapeworms seem to have evolved from parasites of lions and hyenas. The big thing with parasites is the internal environment of a mammal is more or less consistent across species, whereas for viruses the protein coats of various cells can be a bit trickier to break because of inter-specific variation.

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The short answers: Author decides.

There are diseases that are highly specific, and there are others that are more promiscuous.

Malaria is highly specific and will not exist without humans. Yellow fever lives in monkeys and humans. If the other world has monkeys, watch out!

And so on down the scale to rabies that lives in most mammals.

There are many diseases that have variants that can jump species.

Diseases change very rapidly. This means that the diseases of the new world will be different from any disease we have seen on this world. There will be no point in vaccines against known diseases unless you want to make sure your team aren't bringing them along.

On the good news side there will be no antibiotic resistant bacteria. Antibiotics will take care of a lot of diseases.

Basic hygiene, like boiling your water and washing, will prevent a lot of diseases.

After that, it is really up to the author to decide if there is any diseases left that are serious menaces.

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    $\begingroup$ The traveller to a parallel humanless earth who leaves their antimalarials behind might regret it. $\endgroup$ Mar 26 at 19:11
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Who knows? Pathogens evolve very rapidly!

By now we all know how much the viral landscape can change in a short time. Going to a parallel Earth where humans never evolved (or at least didn't survive) means that pathogens have had many thousands of years (or much more) of different evolution that no one can possibly predict. We know that there are many pathogens - rabies, herpes viruses of primates, filariasis ... well, just look up zoonotic diseases! ... which can spread to humans from other animals. But in your world, there's no list to be had, except by filling it out as you learn by experience. As suspicion dawns among your characters that they are at the mercy of your untrammeled creativity, I anticipate a loss of morale.

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