human immunodeficiency virus. See under AIDS virus.
* * *HIV attacks and gradually destroys the immune system, leaving the host unprotected against infection. It cannot be spread through casual contact but instead is contracted mainly through exposure to blood and blood products (e.g., by sharing hypodermic needles or by accidental needle sticks), semen and female genital secretions, or breast milk. A pregnant woman can pass the virus to her fetus across the placenta. The virus first multiplies in lymph nodes near the site of infection. Once it spreads through the body, usually about 10 years later, symptoms appear, marking the onset of AIDS. Multidrug "cocktails" can delay onset, but missing doses can lead to drug resistance. Like other viruses, HIV needs a host cell to multiply. It attacks helper T cells and can infect other cells. A rapid mutation rate helps it foil both the immune system and treatment attempts. No vaccine or cure exists. Abstinence from sex, use of condoms or other means to prevent sexual transmission of the disease, and avoidance of needle sharing have reduced infection rates in some areas.
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