The Human Immunodeficiency Virus (HIV) causes AIDS. The virus attacks the immune system and leaves the body vulnerable to a variety of life-threatening illnesses and cancers. Common bacteria, yeast, parasites, and viruses that ordinarily do not cause serious disease in people with fully functional immune systems can cause fatal illnesses in people with AIDS.

HIV has been found in saliva, tears, nervous system tissue, blood, semen (including pre-seminal fluid, or "pre-cum"), vaginal fluid, and breast milk. However, only blood, semen, vaginal secretions, and breast milk have been proven to transmit infection to others.

Transmission of the virus occurs:

1. through sexual contact -- including oral, vaginal, and anal sex
2. through blood -- via blood transfusions or needle sharing
3. from mother to child -- a pregnant woman can passively transmit the virus to her fetus, or a nursing mother can transmit it to her baby

Other transmission methods are rare and include accidental needle injury, artificial insemination through donated semen, and through a donated organ.

HIV infection is NOT spread by casual contact such as hugging and touching, by touching dishes, doorknobs, or toilet seats, during participation in sports, or by mosquitoes. It is NOT transmitted to a person who donates blood or organs. However, it can be transmitted to the person receiving blood or organs from an infected donor. This is why blood banks and organ donor programs screen donors, blood, and tissues thoroughly.

Those at highest risk include homosexual or bisexual men engaging in unprotected sex, intravenous drug users who share needles, the sexual partners of those who participate in high-risk activities, infants born to mothers with HIV, and persons who received blood transfusions or clotting products between 1977 and 1985 (prior to standard screening for the virus in the blood).

AIDS begins with HIV infection. People who become infected with HIV may have no symptoms for up to ten years, but they can still transmit the infection to others. Meanwhile, their immune system gradually weakens until they are diagnosed with AIDS. Acute HIV infection progresses over time to asymptomatic HIV infection and then to early symptomatic HIV infection and later, to AIDS (very advanced HIV infection):

HIV Infection (acute HIV infection) -->early asymptomatic HIV infection -->early symptomatic HIV infection -->AIDS

Most individuals infected with HIV will progress to AIDS if not treated. However, there is a very small subset of patients who develop AIDS very slowly or never at all. These patients are called non-progressors.

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