The World Health
Organisation's latest estimate is that as many as three million people will develop AIDS in the next five years. Figures such as this, which are calculated by a surprisingly small number of scientists who study the spread of the disease, are the basis of anti-AIDS campaigns all over the world. But how is it possible to predict the development of a new disease about which we still know so little? Geoff Watts examines current ideas about the future of the AIDS epidemic. Producer ROBIN MELHUISH