HIV, or human immunodeficiency virus, attacks the body’s CD4 cells (T cells), which help the immune system fight off infections. If left untreated HIV can lead to the disease AIDS (acquired immunodeficiency syndrome). AIDS is the stage of infection that occurs when your immune system is badly damaged and you become vulnerable to opportunistic infections. A new HIV diagnosis is defined as a diagnosis of HIV infection regardless of the stage of disease (stage 0, 1, 2, 3 [AIDS], or unknown) and refers to all persons with a diagnosis of HIV infection during the the latest year for which data are available. This is a measure of incidence and is not to be confused with HIV prevalence, which is a measure of the number of persons living with diagnosed HIV.