Acquired immunodeficiency syndrome (AIDS) and the entire spectrum of diseases associated with human immunodeficiency virus (HIV) infection have rapidly emerged as major global and national public health problems. About 183,000 cases of AIDS had been reported in 152 countries, as of 30 September 1989, but the actual number is estimated to be over three times as high.8 In addition, there are estimated to be 6 to 8 million who are infected with the virus, but do not yet show major symptoms.  The latest estimates show a continuing and even increasing spread of the disease. WHO's projections indicate that the epidemic will continue to grow throughout the next decade, with about 15 million new infections expected in the early 2000s. Moreover its geographic scope will be much wider, as the disease is already gaining footholds in previously unaffected regions. In countries where it is already prevalent, it is growing in hitherto lightly affected population groups, including children and rural communities.

As the epidemic has been followed for only about seven years, it is not known what proportion of the people infected with HIV will ultimately develop the symptoms of AIDS. Current estimates are that about 50 per cent will develop AIDS within 10 years, but the percentage that will develop AIDS after 15 or 20 years cannot be predicted at this time, nor can the proportions who will eventually die of the disease. The fatality rate among those who develop symptoms appears to be high, but many essential epide-miological characteristics,   including   the   natural   history   of asymptomatic infections, have yet to be elucidated. The cumulative total of AIDS cases world-wide was projected by WHO to exceed 1 million by the early 1990s and could exceed 3 million by the late 1990s."

AIDS is a prolonged, physically debilitating illness that often is economically and emotionally devastating for the victims and their families. Most of the people with AIDS are young and middle- aged adults, whose illness and death deprive their countries of a valuable resource. The number of infants born with HIV infections is increasing, particularly in some developing countries, jeopardizing these countries' recent, hard-won gains in infant and child survival. At present there are no very effective medical techniques for the prevention or treatment of AIDS.  Consequently, educational campaigns to prevent it from spreading are essential while the search for effective treatment continues. The main lines of defence against AIDS are education to reduce high-risk behaviour and provision of a safe blood supply for transfusions, through screening for HIV antibodies. As with most other infectious diseases, a fairly stable prevalence of HIV infections will eventually be established, but how high or low that prevalence will be will depend on the effectiveness of prevention programmes.

The costs of the HIV/AIDS epidemic could prove staggering for both the developed and the developing countries. In the United States, the country with the most reported cases, total federal expenditures on AIDS are projected to exceed $2 billion in fiscal year 1989; the average lifetime costs per AIDS patient in the United States are estimated at $50,000 to $60,000.  Treatment with AZT, the most effective drug available so far for HIV/AIDS, has been costing about $8,000 per patient per year, although some selective reductions in price have recently been offered by the manufacturer. Clearly, such costs are beyond the means of the developing countries, where per capita expenditure on health often does not exceed $5 annually. The enormous cost of caring for AIDS patients threatens to divert resources from other health programmes, with adverse consequences for overall health and mortality.

Although the United States has by far the largest number of reported AIDS cases, several developing countries in the Americas and in central and eastern Africa have a higher incidence of infection. The impact of the disease in these countries will go beyond the normal concern of public health authorities. Associated ethical and humanitarian problems will increase, along with human suffering, and these countries' economic and social development may be held back severely unless treatments are found or a vaccine is discovered in the next few years and made widely available. In some countries in Africa, the incidence of infection is 10 per cent and up among the urban adult population, both male and female, especially those between ages 20 and 50. Workers in this age group are essential in the more modern sectors of the economy, notably in the mining industry in certain African countries. The incidence of infection is also increasing rapidly in several Latin American countries, and is beginning to surge in at least one Asian country (Thailand). Policy- makers in countries whose key industries are about to be seriously affected by AIDS, to a point where they might no longer be internationally competitive, may be forced to use the limited means available to retard the impact of the disease on the workforce in these industries. In the case of the mining industry in Africa, the demographic structure and traditions of health care offer some potential for a comparatively good response to a sustained educational effort.

While it is not yet possible to project the long- term incidence of HIV infections and AIDS cases with much certainty, WHO has used the available information to estimate the impact of AIDS on mortality and population growth in a hypothetical country. This hypothetical case is instructive because the country has characteristics similar to those in some central African areas, where up to 25 per cent of the population 20 to 40 years of age in some cities were infected with HIV in 1987. In the WHO model, the rate at which infected persons progress to AIDS has been estimated to be 20 to 25 per cent within 5 years and close to 50 per cent within 10 years. The progression rate for adults is projected to be 75 per cent within 15 years and 100 per cent within 20 years. The model assumes that half of the infants born to HIV-infected mothers will be infected, and that 80 per cent of infected children will have progressed to AIDS by their fifth birthday. Because of the high infection rates of sexually active females in some urban areas, about 10 per cent of children under five years old in urban areas are assumed to be infected. Persons with AIDS are assumed to die in the same year in which the disease develops. With a population of 20 million and an average HIV infection prevalence of 2.3 per cent in the country as a whole, there would be 450,000 infected people.

In the absence of AIDS, the population would increase by about 6.5 million between 1987 and 1997. Between 1987 and 1997, there would be 479,000 deaths from AIDS, including 320,000 urban residents (187,000 adults and 133,000 children) and 159,000 rural residents. Although the overall effect of AIDS on population growth would be modest in the 10-year period (population growth would be reduced by about 7 per cent over-all), population growth would be 36 per cent less in urban areas. Among the urban population aged 25 to 59 in 1997 (15 to 49 in 1987), the projected population increase between 1987 and 1997 would be 70 per cent less with AIDS; the under-5 age group in 1997 would have grown 50 per cent less than without AIDS. Projections beyond 1997 would depend on the patterns of spread of the HIV infection. If the virus were to continue to increase in urban areas and to spread extensively in rural areas, population growth could turn negative.