The acquired immunodeficiency syndrome AIDS is caused by the retrovirus HIV. About 20% develop after the inoculation of the virus an acute clinical picture resembling infectious mononucleosis. Several weeks to months after the infection antibodies can be demonstrated in the serum. Lateron a lymphadenopathy syndrome or AIDS related complex may develop. Most of the patients with LAS or ARC will progress to the full blown picture of AIDS. This is defined as immunodeficiency complicated by Kaposi-sarcoma or central nervous system malignance lymphoma or opportunistic infections. The most common infections are due to certain parasites, c. e. pneumocystis carinii, toxoplasma gondii and cryptosporidia. Fungi, bacteria and viruses can also cause opportunistic infections.