The real story of AIDS--how it originated with a virus in a chimpanzee, jumped to one human, and then infected more than 60 million people--is very different from what most of us think we know. Recent research has revealed dark surprises and yielded a radically new scenario of how AIDS began and spread. Excerpted and adapted from the book Spillover, with a new introduction by the author, Quammen's hair-raising investigation tracks the virus from...