Heterosexual transmission is the third leading means of acquiring infection.
AIDS primarily affects homo-sexual men, intravenous drug users, recipients of blood products, and the sexual partners and children of people in these groups. The average incubation period is about ten years. The virus that causes AIDS is a member of the family called retroviruses. The entire body of knowledge on retroviruses is derived from research with animals which were found previously to be infected with retroviruses. The identification of the virus that causes AIDS in humans would not have been possible without the animal research that preceded it.