HIV kills or damages cells of the body's immune system. This damage progressively destroys the body's ability to fight infections and certain cancers.
The most advanced stages of HIV infection are known as AIDS. People with AIDS are at great risk of getting very sick from diseases that don't normally affect healthy people. These include viral infections that cause skin tumors and pneumonia, fungal infections of the mouth, lungs, and genitals, and certain cancers. AIDS was first reported in the United States in 1981; it is now recognized that HIV is a major worldwide epidemic.
The National Institute of Allergy and Infectious Diseases (NIAID) has a more comprehensive description of HIV/AIDS.
HIV destroys vital cells of the immune system, called CD4+ T cells. Once enough of these cells are destroyed, the person is considered to have AIDS. This means that the person's immune system has weakened considerably.
A healthy person has between 800 and 1200 CD4+ T cells in one cubic millimeter (about 1/50,000 of a teaspoon) of blood. AIDS occurs when there are fewer than 200 CD4+ T cells per cubic millimeter.
NIAID provides more information about how HIV leads to AIDS.
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