AIDS is caused by the human immunodeficiency virus (HIV). It cannot be caught by sneezing, coughing or casual physical contact. But it can be acquired by unprotected sexual contact with an HIV-positive person; mother-to-child transmission during pregnancy, childbirth or breast-feeding; or exposure to infected blood or blood products—for example, through a blood transfusion, by an accidental needle prick, or by sharing contaminated needles (usually in the case of injection drug use).
Once infected with HIV, an individual is infected for life. Because HIV is a slow-acting virus, most infected people look and feel healthy for years after infection, although they can transmit the virus to others. Without treatment, the time between HIV infection and the development of AIDS-related illnesses varies from six years in Africa to ten years in the Western world. Most patients die within two years after the onset of AIDS. Treatment with antiretroviral drugs can slow the progress of HIV infection—sometimes dramatically. But because there is no cure, most people infected with HIV will eventually die of AIDS-related illnesses.
HIV specifically targets the CD4 cells (or T-helper cells) of the immune system. Once the virus enters the CD4 cell, it replicates itself using the host cell’s genetic machinery. The cache of new HIV particles is then released and goes on to infect more CD4 cells, destroying each one in the process. Since the job of those cells is to mount an immune response against infection, the lower their number, the more vulnerable a person is to infection. A healthy person has a CD4 count of between 500 and 1,000 per cubic milliliter of blood. A person with AIDS has a CD4 count of below 200.
One of the distinguishing features of HIV is that it mutates readily.This has serious implications for patients on antiretroviral therapy. Patients who do not strictly follow their prescribed drug regimen risk developing drug-resistant HIV strains and limiting their treatment options.
AIDS is not a single disease but a cluster of diseases that occur as the immune system fails. Tuberculosis and pneumocystis carinii pneumonia are common in AIDS patients. People with AIDS also have a greater chance of developing cancers, especially those caused by viruses, such as cervical cancer, Kaposi’s sarcoma (a type of skin cancer) and lymphomas. Children with AIDS are additionally susceptible to severe forms of childhood infections like tonsillitis and ear infections.