AIDS was first described in 1981, when previously healthy young adults -- mainly men living in urban areas of the United States -- began falling ill with opportunistic infections previously unknown among this age group. Similar infections were soon described in Africa, the Caribbean and Europe; AIDS was clearly an epidemic disease. Most of these young people died, and a host of discrepant hypotheses emerged, but a bloodborne viral pathogen was suspected early on. In 1983, this suspicion was confirmed when Professor Luc Montagnier and others discovered a novel pathogen: a retrovirus tropic for the CD4 cells that orchestrate cell-mediated immunity and protect humans from a broad range of viral, mycobacterial, and fungal pathogens.

Where effective screening was available, transfusion-associated transmission was eliminated, but HIV was not easily stopped. Condoms were shown to be effective in preventing sexual transmission of HIV, but it was not long before those who studied AIDS concluded that male condoms alone would not be enough in settings in which poverty and gender inequality rendered poor women especially vulnerable to HIV infection (2). Women in turn transmitted HIV to their unborn children or to breastfeeding infants. Injecting drug use introduced HIV to previously untouched regions of the former Soviet Union and to parts of Asia. Poor-quality health care -- including the reuse of syringes, needles, and other medical paraphernalia -- also contributed to the entrenchment of this new epidemic.

The current situation

How well has the international community coped with this new threat to global health? In spite of remarkable scientific achievements -- the development of inexpensive diagnostics by the mid-1980s, the sequencing of the entire HIV genome less than 15 years later, and the development of effective antiretroviral therapy by 1995 -- the virus has continued to spread (see Figure 3.2). It is estimated that during the course of 2002 some 5 million people became infected with HIV, and almost 3 million people died of AIDS.

Figure 3.2

Everywhere in the world, HIV is transmitted through a fairly limited number of mechanisms. HIV is a bloodborne retrovirus and is transmitted through sexual contact, contaminated blood transfusions, injecting drug use, failure to observe what are now termed in medical circles "universal precautions", and from mother to infant during pregnancy, delivery and breastfeeding.

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The most heavily burdened continent is Africa, where the spread of the pandemic has been accelerated by a variety of factors, including widespread poverty, gender inequality, and health systems weakened by pressures such as the large external debt loads of states. Africa is home to more than 70% of those currently infected with HIV. Of all AIDS deaths worldwide -- 28 million at the end of 2002 -- the majority have also occurred on this continent (3). HIV infection has fanned epidemics of TB in some African countries, increasing the risks to the whole population, regardless of serostatus. Across sub-Saharan Africa, rates of TB have more than trebled, and many conclude that the disease cannot be controlled without aggressive treatment of AIDS (4).

Debates about the relative importance of different modes of HIV transmission in sub-Saharan Africa persist, but the evidence indicates that HIV in this region is primarily a sexually transmitted pathogen (5). Nonetheless, the difficulties involved in following universal precautions in overburdened and under-resourced health care facilities may lie at the root of many AIDS deaths in Africa (6). In many regions, unsafe blood transfusions continue, underscoring the importance of blood safety as a component of effective HIV/AIDS control.

The immensity and rapidity of the spread of HIV have reversed gains in life expectancy in many African countries (see Chapter 1). But the worst may be yet to come. The poorer regions of Asia, including densely populated southern Asia, are the latest areas to be affected by the emerging AIDS epidemic. There has been an alarming rise in HIV/AIDS cases in Asia over the past two decades; the burden of disease and death in the region will be enormous if current epidemiological trends are not slowed or reversed.

Developed countries are also afflicted. The Russian Federation and Ukraine, along with other countries in eastern Europe and countries in central Asia, have the most rapidly expanding HIV epidemics. Here the disease is more closely tied to injecting drug use, which itself is linked to a rapid rise in indices of social inequality (3). Although the absolute number of AIDS cases in the former Soviet Union remains relatively small, the epidemic is expanding rapidly in the Russian Federation and other countries in the region (see Box 3.1). Prison-seated epidemics of TB, including drug-resistant strains, will be further fanned by the rapid rise in HIV incidence already documented among Russian prisoners. Only aggressive harm-reduction efforts and improved therapy for TB can stem what is a peculiarly modern epidemic of two linked diseases that are now colliding within countries reaching from western Europe to the Pacific Ocean (7).

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Box 3.1 HIV/AIDS in the European Region

The European Region is experiencing the fastest growing HIV epidemic in the world, and significant further growth is likely. Between 1995 and 2003, the number of newly reported HIV infections in western Europe doubled to almost 170 000, and in central and eastern Europe grew from 27 000 to 320 000. It is now estimated that at least 1.7 million people in Europe are already infected with HIV.

An epidemic of injecting drug use is fuelling the HIV epidemic. In the former Soviet Union, where two-thirds of all Europeans infected with HIV live, 84% of all HIV cases with a known transmission route are attributable to injecting drug use. In western Europe, sexual transmission is the dominant route, with the largest number of infections among men who have sex with men and among immigrant populations from high prevalence countries.

The HIV/AIDS epidemic has changed dramatically since its onset in the 1980s. Sustainable, long-term, comprehensive and massive response in western Europe, with extensive funding and political support for prevention, treatment and care limited the spread of the epidemic. Widespread prevention measures contributed to stabilizing and decreasing transmission rates, while antiretroviral treatment was also made widely available. In spite of such measures, the HIV/AIDS epidemic remains a serious and ongoing challenge in western Europe. In eastern Europe, limited political commitment and funding, low coverage by prevention services, severely limited access to treatment and care and high levels of stigmatization of groups at risk are the main reasons for a continuously worsening HIV epidemic.

The true toll of HIV/AIDS

The epidemiology of HIV, including the dynamics of risk, is instructive. But epidemiology tells only part of the story. A disease that has so adversely affected life expectancy will also take a great toll on the social fabric of heavily burdened societies, above and beyond its terrible cost in immediate human suffering. To take two examples, AIDS has orphaned an estimated 14 million African children and will decimate the corps of teachers, health care workers and civil servants in the hardest-hit countries (8).

These social disruptions are manifest in direct economic effects. A recent World Bank study predicts that South Africa will face "complete economic collapse … within three generations" if the country does not take effective measures to combat AIDS (9). But there is much more to the story than can be measured by economists. Other social scientists are broadening the analysis to look at the impact of the disease on an array of events and processes. What, for example, are the social consequences of having millions of AIDS orphans? How does the disappearance of so many parents contribute to the spread of armed conflict and the social pathologies that accompany urban migration and attendant unemployment? What is the cost of the "burnout" registered among health professionals across Africa and in other settings where AIDS therapy is not available to those who need it most (10)? Science is only beginning to understand the social and economic toll of HIV/AIDS, which is heaviest in precisely those settings least prepared for a new threat to health and well-being.