In 2004, the World Health Organization (WHO) estimated that there were 39.4 million people living with HIV/AIDS, 4.9 million new infections and 3.1 million deaths. The cumulative death toll since the epidemic began is over 20 million, the vast majority of cases occurring in sub-Saharan Africa where over 13 million children have been orphaned.

In Botswana, South Africa, Swaziland and Zimbabwe, 25-40% of adults are infected, with rates of 5-15% in most other sub-Saharan African countries. A total of 64% of all persons with HIV live in sub-Saharan Africa, with South Africa accounting for one-third of AIDS deaths globally. Since 2002, the steepest increases have been seen in East Asia (50%), attributable largely to the epidemic occurring in China, and in Eastern Europe and Central Asia (40%), where alarming increases have been seen in the Ukraine, Latvia and Russia. In Vietnam, Thailand, Cambodia, Nepal and Myanmar, HIV is now well established. In India, it is estimated that 5.1 million persons are currently infected; in Tamil Nadu district 50% of sex-workers are infected, and in Manipur over 5% of pregnant women are positive. In northern India, HIV is well established in injection drug-users, of whom 25-50% is infected. Given that Asia is home to 60% of the world’s population, these changes have huge implications.

Many different cultural, social and behavioural aspects determine the regional characteristics of HIV disease. In the USA and northern Europe, the epidemic has predominantly been in men who have sex with men, whereas in southern and Eastern Europe, Vietnam, Malaysia, North-east India and China the incidence has been greatest in injection drug-users. In Africa, the Caribbean and much of South-east Asia the dominant routes of transmission are heterosexual (sex between man and woman) and from mother to child (vertical). The economic and demographic impact of HIV infection in developing countries is profound as it affects the most economically productive and fertile ages and is also eroding the health and economic advances made in the last few decades. Unlike the situation in developed nations, fewer than 5% of patients in resource-poor countries are able to access antiretroviral drugs.

The epidemic in industrialized nations is changing. Heterosexual transmission has become the dominant route, with racial and ethnic minorities representing an increasing fraction. Around 58% of infections in the UK in 2003 were acquired heterosexually and 75% of these were acquired abroad in a country with a high prevalence of HIV, mainly sub-Saharan Africa. High-risk sexual behaviour is also increasing in many developed countries, including in persons aware of their HIV-positive status.