PIP: AIDS has spread worldwide regardless of geographic barriers, race, age, or gender. The most important routes of HIV transmission are sexual contact; exposure to blood, such as through IV drug use and transfusion; and perinatal transmission from infected mothers to their infants. This paper reviews the literature on HIV transmission through blood and other tissues, semen and HIV transmission, the sexual transmission of HIV, and vertical transmission. The sexual transmission of HIV is considered in subsections on the frequency of sexual intercourse, genital and anorectal ulcers, other factors in sexual practices, and therapeutic parameters. Vertical transmission is considered in subsections on the pathophysiological mechanisms, diagnostic technology and pediatric AIDS, factors influencing vertical transmission, and breast feeding and HIV transmission.