PIP: Although the acquired immunodeficiency syndrome (AIDS) pandemic is still in its early stages and its ultimate dimensions are difficult to gauge, it is clear that AIDS represents an unprecedented threat to global health. Over 250,000 cases of AIDS have already been reported and another 10 million people are infected with the AIDS virus, suggesting that about 1 million new AIDS cases can be expected within the next 5 years. Studies have consistently shown that the human immunodeficiency virus (HIV) is transmitted by anal or vaginal sexual intercourse, by the injection or administration of infected blood, and from an infected mother to her infant. An analysis of seroprevalence data suggests 3 distinct patterns of the disease. Pattern 1, typical of large industrialized countries such as the US, includes a male-to-female sex ratio of reported AIDS cases of about 10-15 to 1 and a concentration of cases among homosexual/bisexual males and intravenous drug users in urban areas. Pattern 2 is presently observed in some areas of central, eastern, and southern Africa and increasingly in the Caribbean. Most cases in pattern 2 areas occur among heterosexuals and there is a 1 to 1 ratio between infected males and females. Given the high numbers of women affected, perinatal transmission is common. Pattern 3 prevails in areas of Eastern Europe, North Africa, the Middle East, Asia, and most of the Pacific. In these areas, only a small number of AIDS cases have been reported and most of these involve individuals who have travelled to pattern 1 or 2 countries. Africa, where all 3 infection patterns can be found, has been hardest hit by the AIDS epidemic. From 10-25% of women of childbearing age are infected with HIV, implying a 25% increase in child mortality in the years ahead. The cumulative total of AIDS cases in Africa was estimated at over 100,000 in mid-1988 and an additional 400,000 cases are projected in the next 5 years. Studies in Africa suggest that the presence of other sexually transmitted diseases, and an immune system already compromised by chronic diseases, increases the risk of HIV infection. The control of AIDS requires a sustained, longterm national and international effort.