How did the ancestors of microbes now confined to us transfer themselves from their original animal hosts in Guns, Germs, and Steel?From Chapter 11.

Diamond argues that the move from animals to humans came
in four stages.  This starts on p. 207 in my book -- about 14 pages in to the
chapter.

First, there are diseases that come directly to
people from animals.  This can happen, for example, if a microbe enters a person as they
butcher an animal.

Second, the animal pathogen can evolve
and then get transmitted.  Diamond says that in this stage, the epidemic dies out for
one reason or another.  An example of this was kuru, which ended when cannibalism in New
Guinea ended.

Third, you can have pathogens that evolve,
cross over, and do not die out.  That seems to be what has happened with
AIDS.

Fourth, there are microbes that have moved over to us
and that cannot make it back to animals.