In North America and Europe during the 1980s and early 1990s, HIV was transmitted predominantly through unprotected sexual intercourse among MSMs (male-to-male sexual contacts) and through IDU with contaminated needles. During the late 1990s heterosexual intercourse and IDU became the prevailing modes of HIV transmission in North America and Europe.

In sub-Saharan Africa the overwhelming mode of transmission has been heterosexual intercourse. In this part of the world transmission through MSM contact or through IDU is slight. Because many women have been infected, rates of perinatal transmission are increasing. UNAIDS reports in 2006 Report on the Global AIDS Epidemic that 90% of HIV-positive babies born worldwide in 2004 were in sub-Saharan Africa. The observation that the prevalence of HIV infection has been rising in young pregnant women--doubling between 2000 and 2004--suggests that new infections continue to increase and may point to further growth in the region's epidemic.

The rates of MSM transmission in Latin America are similar to those of Europe and the United States, but IDU is less frequent, whereas heterosexual transmission is considerably higher. In South and Southeast Asia the rapid increase of HIV can be traced to shared contaminated injection equipment and heterosexual intercourse. Half the new HIV infections in China in 2005 occurred during unprotected sex. Similarly, even though most HIV infections in India are occurring because of unprotected heterosexual intercourse, contaminated injection drug equipment is also a significant risk factor for HIV infection in northeast India.

In other areas the highest national HIV infection levels in Asia continue to be found in Southeast Asia, where combinations of unsafe practices with sex workers and MSM, along with IDU, continue to fuel and maintain the epidemics. Even though the HIV infection rate has peaked in other parts of the world, it is escalating in Asia, mainly from heterosexual intercourse through prostitution. Analysis of transmission patterns also reveals significant epidemics among MSM in Cambodia, China, India, Nepal, Pakistan, Thailand, and Vietnam.

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