Human immunodeficiency virus (HIV) is the virus that causes acquired immune deficiency syndrome (AIDS), a chronic life-threatening condition that damages the cells of the immune system. This prevents the body from fighting off viruses, bacteria, fungi and parasites. People infected with HIV and AIDS are vulnerable to the damaging effects of many diseases, including infections and some types of cancer. There is no known cure for HIV or AIDS.

HIV and AIDS are part of the same disease, with HIV occurring in the early stages of the disorder and AIDS occurring in the later stages. There are several strains of HIV. The most common strain is HIV-1, but a person can become infected with more than one strain.

The HIV virus spreads through a variety of body fluids, including:

• Blood
• Semen
• Vaginal fluids
• Breast milk
• Body fluids handled by healthcare workers, including amniotic fluid

HIV infection attacks white blood cells known as CD4 lymphocytes, which coordinate the efforts of the immune system in attacking disease. Normally, white blood cells and antibodies attack and destroy foreign organisms (antigens) in the body. HIV attaches to CD4 lymphocytes and “hijacks” them by inserting its own genetic material into the cells and replicating itself.

Eventually, these copies of the virus enter the bloodstream and begin attacking other cells. When this occurs, the original CD4 lymphocyte that spawned the copy dies. The cycle repeats, with more than 10 billion new HIV particles produced every day. The immune system reacts by trying to create vast quantities of new CD4 cells – as many as 2 billion new cells each day – but eventually finds itself overpowered by the virus.

As the virus continues to overwhelm the immune system, the number of CD4 cells in the body decreases. This results in severe immune deficiency, in which the body is left unable to fight off viruses and bacteria. Eventually, the disease progresses to the point where it becomes classified as AIDS.

The U.S. Centers for Disease Control and Prevention (CDC) defines AIDS as the presence of HIV infection (having a positive test for the HIV virus) and one or both of the following:

• CD4 lymphocyte count of 200 or less. Normal counts range from 600 to 1,000.
  
  
• Development of an opportunistic infection. This is an infection that occurs when the immune system is impaired. Patients infected with HIV are susceptible to various viral, bacterial, fungal and parasitic infections. They are also more likely to develop certain cancers, including:
  
  
• Kaposi's sarcoma. A type of cancer that starts with purplish spots on the feet and legs and spreads from the skin to lymph nodes and internal organs.
  
  
• Lymphoma. Malignant (cancerous) tumors that arise in the lymph nodes or in other lymphoid tissue.