How did HIV spread from chimpanzees to humans?

Due to the close genetic relationship between nonhuman primates and humans, disease causing organisms are easily exchanged between them. The pathogens that can be passed from nonhuman primates to humans and vice versa include bacteria, fungi, parasites, and viruses.

They may be spread by bites, scratches, handling animals or their tissues, airborne transmission of aerosols and droplets, ingestion, and arthropod vectors. Often the nonhuman primate carries and transmits disease without any visible signs. Persons in contact with these animals must always be aware of the potential risks involved.

This is especially true when animals are under stress, such as those that have been recently shipped or introduced into a new situation, or have developed a recent illness. As with many communicable diseases, immunocompromised persons are at greatest risk for infection or serious consequences from such infections.