Within 3 years after the syndrome was recognized in the United States, cases of AIDS were reported from every continent. By November of 1997, more than 1.7 million cases had been reported from 197 countries to WHO (Table 409-4) . WHO also estimates that >60 milion people would be infected with HIV by the end of century. 

AIDS case reporting from developing countries is much less complete than in industrialized countries and, because the developing world is most heavily affected by HIV/AIDS, WHO estimates that the number of reported AIDS cases worldwide are less than 15% of the total estimated number of AIDS cases that have occurred throughout the epidemic. Extensive HIV serosurveys in Africa, South and Central America, and parts of Asia provide evidence that AIDS case reports greatly underestimate the magnitude of the HIV problem in many countries in these regions.

Modes of transmission of HIV are similar throughout the world, but the relative frequency varies considerably between countries and regions. In North America, Europe, Australia, New Zealand, and some areas of South America, the majority of HIV infections first occurred in homosexual men and IDUs; heterosexual and perinatal transmission initially resulted mostly from transmission from IDUs and their partners. In most countries in Africa and some in the Caribbean and Central America, most HIV infections have occurred through heterosexual transmission.

HIV seroprevalence rates are highest in urban prostitutes and sexually active young adults. High rates of infection in young women translate into a substantial amount of perinatal transmission. In some areas of Africa, pediatric HIV infection has significantly increased already high infant mortality rates. In many developing countries, transfusion of HIV-infected blood remains a substantial problem because of inadequate blood banking and serologic testing capacity. Reuse of nonsterile needles and syringes and other medical practices have caused major HIV outbreaks in the former Soviet Union and Romania. In Asian countries such as Thailand and India, emergence of HIV infection as a major public health problem began in IDUs and prostitutes, but rapidly spread more widely through heterosexual transmission to other young adult populations. India is now the country believed to have the highest number of HIV-infected persons, with an estimated 2 to 5 million people currently infected.

In Eastern Europe and the newly independent states of the former Soviet Union, HIV infections are rapidly increasing, primarily in association with injecting drug use. In yet other countries, primarily in the Middle East, Asia, and the Pacific region, HIV has not yet been recognized as an important public health problem. The future course of HIV in these countries may depend on their ability to anticipate and respond to the problem; it can be approximately predicted by the extent and pattern of sexually transmitted and transfusion-associated infections and the extent of injecting drug use that currently exists in each country.

A second human immunodeficiency virus, HIV type 2 (HIV-2), was first described in asymptomatic West Africans with AIDS in 1986. HIV-2 infection remains most prevalent in West Africa, although well-documented cases have been reported from Western Europe, Canada, Brazil, the United States, and Central Africa. HIV-2 is generally less virulent than HIV-1. The average viral titer usually is lower, perhaps explaining the lower rates of sexual and perinatal transmission and the slower rate of disease progression in persons infected with HIV-2 than HIV-1. HIV-1 and HIV-2 are closely related; tests for antibody for one virus often cross react with those for the other.

For example, licensed enzyme immunoassays for detecting HIV-1 find HIV-2 antibody in 60 to 90% of infected patients. In the United States, combined HIV-1/HIV-2 assays are used to test donated blood. As of 1994, HIV-2 infection remained rare in the United States, with nearly all cases detected in persons from West Africa.

Recently, additional HIV variants, classified together as subtype O, were reported from Cameroon. The antibody response elicited by group O strains is not consistently detected by enzyme immunoassay (EIA) kits commercially available in Europe and the United States. In 1996, a patient with HIV-1 group O infection was diagnosed in California, which reinforces the need for strong international collaboration in maintaining surveillance for variants of HIV and other emerging infections.

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Force Recommendations for the Use of Antiretroviral Drugs in Pregnant Women Infected with Human Immunodeficiency Virus-1 for Maternal Health and for Reducing Perinatal HIV-1 Transmission in the U.S. MMWR 47 (RR-2):1, 1998. These summarize available information on factors related to heterosexual transmission of HIV and prevention of perinatal transmission.

Mann JM, Tarantola DJM (eds): AIDS in the World II Global Dimensions, Social Roots and Responses. New York: Oxford University Press, 1996. This volume summarizes and updates what is known about HIV infection and prevention activities throughout the world.
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