AIDS (Acquired Immune Deficiency Syndrome) is the final and most serious stage of HIV disease, which causes severe damage to the immune system.

According to the Centers for Disease Control and Prevention, AIDS begins when a person with HIV infection has a CD4 cell count below 200. (CD4 is also called "T-cell", a type of immune cell.) AIDS is also defined by numerous opportunistic infections and cancers that occur in the presence of HIV infection.

AIDS is the fifth leading cause of death among persons between ages 25 and 44 in the United States. About 47 million people worldwide have been infected with HIV since the start of the epidemic.

Human immunodeficiency virus (HIV) causes AIDS. The virus attacks the immune system and leaves the body vulnerable to a variety of life-threatening infections 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), 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 (now extremely rare in the U.S) or needle sharing
3. From mother to child -- a pregnant woman can transmit the virus to her fetus through their shared blood circulation, or a nursing mother can transmit it to her baby in her milk

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

HIV infection is not spread by casual contact such as hugging, by touching items previously touched by a person infected with the virus, during participation in sports, or by mosquitoes.

It is not transmitted to a person who DONATES blood or organs. Those who donate organs are not in direct contact with those who receive them. Likewise, a person who donates blood is not in contact with the person receiving it. In all these procedures, sterile needles and instruments are used.

However, HIV can be transmitted to a 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 persons engaging in unprotected sex, the sexual partners of those who participate in high-risk activities (such as anal sex), intravenous drug users who share needles, infants born to mothers with HIV, and people 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 infected with HIV may have no symptoms for ten years or longer, but they can still transmit the infection to others during this symptom-free period. Meanwhile, if the infection is not detected and treated, the immune system gradually weakens and AIDS develops.

Acute HIV infection progresses over time to asymptomatic HIV infection and then to early symptomatic HIV infection. Later, it progresses to AIDS (very advanced HIV infection with T-cell count below 200).

Most individuals infected with HIV, if not treated, will develop AIDS. There is a small group of patients who develop AIDS very slowly, or never at all. These patients are called non-progressors and many seem to have a genetic difference which prevents the virus from attaching to certain immune receptors.