Transmission through sexual contact accounts for 75 to 85 percent of the nearly 28 million infections with the human immunodeficiency virus (HIV) that have occurred so far.¹ The probability of infection through sexual contact, although it varies greatly, appears to be lower than that of infection through other routes of exposure (Figure 1). The variability observed among and within routes of HIV exposure depends partly on the viral dose and also on whether the virus is transmitted directly into the blood or onto a mucous membrane. In addition, these differences are influenced by a variety of host factors, including both . . .