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.)