HIV has infected 40,000,000 people, killed >6,000,000 people, and devastated whole sectors of societies [1]. HIV infection is rightly considered by the United Nations to be a major threat to global security and economy [2]. Prevention of the spread of HIV demands precise knowledge of the biology and epidemiology of transmission [3], a challenge that even now has not been completely met. Although several studies of HIV epidemiology (reviewed in [4]) have described heterosexual transmission as occurring in ∼1/1000 coital acts, this number seems far too low to explain the magnitude of the HIV pandemic