AIDS is a transmissible immunodeficiency syndrome which has first been observed less than a decade ago and since that time has spread in an epidemic manner. Usually it manifests itself by opportunistic infections and/or neoplasias. After courses of a few years, approximately 100% of cases have a lethal outcome. Sometimes, neuropsychiatric disturbances are the presenting symptoms and signs of AIDS. One of the first important observations was, that certain behavioural patterns such as homosexuality and intravenous drug abuse were apparently associated with a high risk for acquiring AIDS. The occurrence of AIDS in a number of haemophiliacs and recipients of blood transfusions suggested an important role of the haematogenous route of transmission. Not more than two years after the first clinical reports on AIDS a retrovirus was identified as the etiological agent. On the basis of different criteria this retrovirus can be classified as a slow virus. Subsequently, virological tests were developed which allowed an early diagnosis of this viral infection, even prior to the evolution of clinical symptoms. Immunological features of this new syndrome include disturbances of cellular as well as humoral immune functions. As we have no effective chemotherapy for AIDS and the successful development of a vaccine is delayed by a number of virological problems, it is of special importance to prevent transmission of the disease.