AIDS Acquired immune deficiency syndrome, or AIDS, is a recently recognized disease entity. It is caused by infection with the human immunodeficiency virus (HIV), which attacks selected cells in the immune system (see IMMUNITY) and produces defects in function. These defects may not be apparent for years. They lead in a relentless fashion, however, to a severe suppression of the immune system's ability to resist harmful organisms. This leaves the body open to an invasion by various infections, which are therefore called opportunistic diseases, and to the development of unusual cancers. The virus also tends to reach certain brain cells. This leads to so-called neuropsyc