Monday marks the 25th anniversary of the first report of AIDS. But only recently have scientists come to conclusions about where HIV came from. The current thinking is that the colonial horrors of mid-20th-century Africa allowed the virus to jump from chimpanzees to humans and become established in human populations around 1930. But there is still uncertainty as to why AIDS was first discovered in Los Angeles and New York, and not Cameroon, where scientists say it surely started.