Acquired immunodeficiency syndrome
(AIDS) is an illness that weakens the body's immune system. The immune system of a person with AIDS is not able to fight off certain infections and cancers.
The Immune System
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AIDS is caused by the human immunodeficiency virus (HIV), which destroys important immune system cells. HIV is spread through contact with HIV-infected blood or other body fluids, including semen, vaginal fluid, and breast milk. Unprotected sex is the most common mode of transmission. Sharing needles for injecting drugs is another common way the virus is transferred.