The Human Immunodeficiency Virus (HIV) attacks the immune system and weakens defenses against many infections and some types of cancer. The virus destroys and weakens the function of immune cells, so that infected people gradually develop immunodeficiency. Immune function is usually measured by the number of CD4 cells.
The most advanced stage of HIV infection is acquired immunodeficiency syndrome (AIDS), which may develop many years later in some people if left untreated. AIDS is characterized by the development of certain types of cancer, infections, or other severe clinical manifestations of a chronic nature.
