The origin of acquired immune deficiency syndrome (AIDS) and human immunodeficiency virus (HIV) has puzzled scientists ever since the illness first came to light in the early 1980s. For over 25 years it has been the subject of fierce debate and the cause of countless arguments. It is now generally accepted that HIV is a descendant of a simian immunodeficiency virus and there are many theories about how this ′zoonosis′ originated and how SIV became HIV in humans. Advances in treatment have steadily reduced the morbidity and mortality associated with HIV infection. However, in the low-income, high-prevalence countries, antiretroviral medication has taken a long time to reach the people who actually need it. Access to medication must greatly improve if millions of deaths are to be avoided. HIV is a preventable disease. Unless great progress is made in prevention, the number of people living with HIV will outstrip the resources available for treatment. The search for effective vaccines and microbicides must therefore be one of the very highest priorities. HIV is a global threat. Action needs to be taken to prevent it killing many more millions than those who have already died. This action needs not only to continue, but to be speeded up considerably.