In Africa, reporting has only recently started in some countries and is, therefore, incomplete. Consequently, the proportion of AIDS cases that are reported in Africa is unknown. Epidemiologic studies indicate three broad yet distinct geographic patterns of transmission. 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. IV drug use and homosexual transmission either do not occur or occur at a very low level. Transmission through contaminated blood and blood products has been a significant problem and continues in those countries that have not yet implemented nationwide donor screening. 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.