Human immunodeficiency virus, or HIV, is the virus that causes acquired immune deficiency syndrome (AIDS). The virus weakens a person's ability to fight infections and cancer. People with HIV are said to have AIDS when they develop certain infections or cancers or when their CD4 count is less than 200. CD4 count is determined by a blood test in a doctor's office.
HIV attacks and destroys a type of white blood cell called a CD4 cell. This cell's main function is to fight disease. When a person's CD4 cell count gets low, they are more susceptible to illnesses. AIDS is the final stage of HIV infection. When the immune system CD4 cells drop to a very low level, a person's ability to fight infection is lost. In addition, there are several conditions that occur in people with HIV infection with this degree of immune system failure -- these are called AIDS-defining illnesses.
According to the CDC, 1,051,875 people in the U.S. have been diagnosed with AIDS since the disease was first diagnosed in 1981. They also estimate that 583,298 have died from the disease in the U.S.