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 cells are also called "T-cells" or "helper cells"; they are 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, down from number one in 1995. About 25 million people worldwide have died from this infection since the start of the epidemic, and 40.3 million people are currently living with HIV/AIDS globally

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 healthy immune systems can cause fatal illnesses in people with AIDS.

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

Transmission of the virus occurs:

• Through sexual contact -- including oral, vaginal, and anal sex
• Through blood -- via blood transfusions (now extremely rare in the U.S) or needle sharing
• 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 organ transplants.

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
• 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 who don't receive HIV therapy during pregnancy
• People who received blood transfusions or clotting products between 1977 and 1985 (prior to the beginning 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 (defined as 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 that prevents the virus from attaching to certain immune receptors.

The symptoms of AIDS are primarily the result of infections that do not normally develop in individuals with healthy immune systems. These are called opportunistic infections.

Patients with AIDS have had their immune system depleted by HIV and are very susceptible to such opportunistic infections. Common symptoms are fevers, sweats (particularly at night), swollen glands, chills, weakness, and weight loss.

See the signs and tests section below for a list of common opportunistic infections and major symptoms associated with them.

Note: Initial infection with HIV can produce no symptoms. Most people, however, do experience flu-like symptoms with fever, rash, sore throat, and swollen lymph nodes, usually two weeks after contracting the virus. Some people with HIV infection remain without symptoms for years between the time of exposure and development of AIDS.

The following is a list of AIDS-related infections and cancers that people with AIDS acquire as their CD4 count decreases. Previously, having AIDS was defined as having HIV infection and getting one of these additional diseases. Now it is additionally defined as a CD4 count below 200, even without an opportunistic infection. Many other illnesses and corresponding symptoms may develop in addition to those listed here.

Common with CD4 count below 350 cells/ml:

• Herpes simplex virus -- causes ulcers/vesicles in the mouth or genitals, occurring more frequently and more severely in an HIV-infected patient than before HIV infection
• Tuberculosis -- infection by the tuberculosis bacteria that predominately affects the lungs, but can affect other organs such as the bowel, lining of the heart or lungs, brain, or lining of the central nervous system
• Oral or vaginal thrush -- yeast infection of the mouth or genitals
• Herpes zoster (Shingles) -- ulcers/vesicles over a discrete patch of skin caused by the varicella zoster virus
• Non-Hodgkin's lymphoma -- cancer of the lymph glands
• Kaposi's sarcoma -- Cancer of the skin, lungs, and bowel, associated with a herpes virus (HHV-8). Can occur at any CD4 count, but more likely at lower CD4 counts, and more common in men than women

CD4 count below 200 cells/ml

• Pneumocystis carinii pneumonia, "PCP pneumonia," now called Pneumocystic jiroveci pneumonia
• Candida esophagitis -- painful yeast infection of the esophagus
• Bacillary angiomatosis -- Skin lesions caused by a bacteria called Bartonella, which is usually acquired from cat scratches

CD4 count below 100 cells/ml

• Cryptococcal meningitis -- infection of the lining of the brain by a yeast
• AIDS dementia -- worsening and slowing of mental function, caused by HIV itself
• Toxoplasmosis encephalitis -- infection of the brain by a parasite, which is frequently found in cat feces; causes discrete lesions in the brain
• Progressive multifocal leukoencephalopathy -- a viral disease of the brain caused by a virus (called the JC virus) that results in a severe decline in cognitive and motor functions
• Wasting syndrome -- extreme weight loss and loss of appetite, caused by HIV
• Cryptosporidium diarrhea -- Extreme diarrhea caused by one of several related parasites

CD4 count below 50/ml

• Mycobacterium avium -- a blood infection by a bacterium related to tuberculosis
• Cytomegalovirus infection -- a viral infection that can affect almost any organ system, especially the large bowel and the eyes

In addition to the CD4 count, HIV RNA load, and basic screening lab tests, regular vaginal Pap smears are important to monitor in HIV infection, due to the increased risk of cervical cancer in immunocompromised patients. . Anal Pap smears to detect potential cancers may also be important in both HIV infected men and women.