Epidemiologic studies indicate three broad yet distinct geographic patterns of transmission.

Pattern I is typical of industrialized countries with large numbers of reported AIDS cases, such as North America, Western Europe, Australia, New Zealand, and parts of Latin America. In these areas, most cases occur among homosexual or bisexual males and urban IV drug users. Heterosexual transmission is responsible for only a small percentage of cases but is increasing.

Pattern II is observed in areas of central, eastern, and southern Africa and in some Caribbean countries. In these areas, most cases occur among heterosexuals; the male to female ratio is approximately 1:1 and perinatal transmission is relatively more common than in other areas. IV drug use and homosexual transmission either do not occur or occur at a very low level.

Pattern III is found in areas of Eastern Europe, the Middle East, Asia, and most of the Pacific. HIV appears to have been introduced into these areas in the early to mid-1980s, and only small numbers of cases have been reported. Homosexual and heterosexual transmission has only recently been documented. Generally, cases have occurred among persons who have traveled to endemic areas or who have had sexual contact with individuals from endemic areas, such as homosexual men and female prostitutes.