AIDS is the final stage in a process that begins when a person is infected with the human immunodeficiency virus, or HIV. The virus then launches a slow, relentless attack on the immune system and other cells throughout the body.

HIV is transmitted through sexual intercourse. It is also spread through use of contaminated needles or from an infected mother to an infant. Because of screening tests instituted in 1985, the blood supply in the U.S. is considered generally safe.

Once the virus enters the bloodstream, it attacks and destroys a key type of infection-fighting white blood cell in the immune system -- a T cell. Most healthy people have a T cell count of at least 800. As the level of T cells drops and the immune system begins to fail, the patient is more easily infected by unusual viruses, bacteria and fungi. Once the T cell count drops below 200, the person is at high risk of dying from an AIDS-related disorder.

In some cases, as long as 10 years elapse from the time of infection with HIV to the development of symptoms; in other cases, people get sick much sooner.

The diagnosis of AIDS is made when an AIDS-related infection occurs. These include pneumonia, encephalitis and a rare skin cancer called Kaposi's sarcoma.

Several drugs are now in use that slow the progression of the AIDS virus -- such as AZT -- or treat or prevent some of the infections and cancers associated with AIDS.


1. HIV attaches to the surface of a T cell.

2. The virus enters the cell and unpacks its genetic material in the chemical form of RNA.

3. The genetic material is converted to the more common form of DNA.

4. The DNA of the AIDS virus mixes with the cell's DNA so that it now dominates the cell.

5. The AIDS genes can remain dormant or start making more copies of the AIDS virus.

Many doctors recommend starting AZT treatment when T cell count drops below 500


1. The AIDS virus attacks the immune system directly by 1. entering a cell, reproducing more virus and damaging the cell's membrane so that the cell dies. It attacks indirectly by 2. causing two uninfected T cells to stick together so that they are immobilized; or 3. provoking normal immune cells to attack and kill an already infected T cell -- further weakening the immune system.


Most healthy people have a T cell count of at least 800. As HIV attacks the immune system, the body's T cell count begins to drop, leaving the person vulnerable to a range of other infections and cancers.Once the T cell count drops below 200, the person is at high risk of dying from an AIDS-related disorder.


The earlier infection is detected, the sooner treatment can begin to slow the progress of the virus. The most common blood test looks for antibodies produced by the immune system in response to the HIV virus. Antibodies may appear as early as six weeks; typically, they appear within six months. During that time, a person may not be aware of the infection and can pass the virus on to others.

After 18 months, another test can detect the actual virus in the blood.


HIV can infect: 1. the brain 2. bone marrow 3. lungs 4. skin 5. lymph nodes 6. colon-rectum 7. blood 8. semen and vaginal fluids


Scientists are working on ways to interfere with the life cycle of the AIDS virus by: 1. Blocking the virus from binding to a cell. 2. Inhibiting the conversion of the AIDS virus's RNA into DNA. 3. Preventing the virus from copying itself and multiplying.

The drug AZT has been shown to prolong the lives of people with AIDS and slow the progression from HIV infection to AIDS. Other drugs are also being used and tested.


A person can die from one or more of the infections and cancers that occur after the immune system is weakened:

Cytomegalovirus, a cause of blindness and pneumonia

Kaposi's sarcoma, a skin cancer that produces purplish blotches

Lymphoma, a blood cancer


Toxoplasmosis, an intestinal parasite that attacks the brain and causes dementia

Encephalitis, an inflammation of the brain