AIDS

Acquired immune deficiency syndrome, a deficiency of the immune system due to infection with HIV (human immunodeficiency virus). In most countries, illness and death from AIDS is a growing health problem, and there is, as yet, no cure or vaccine. AIDS does not develop in all people infected with HIV. The interval between infection and the development of AIDS is highly variable. Without treatment, around half of those people infected will develop AIDS within 8–9 years. HIV is transmitted in body fluids, including semen, blood, vaginal secretions, and breast milk. The major methods of transmission are sexual contact (vaginal, anal, or oral), blood to blood (via transfusions or needle-sharing in drug users), and mother to fetus. HIV has also been transmitted through blood products given to treat haemophilia, artificial insemination by donated semen, and kidney transplants; but improved screening has greatly reduced these risks. HIV is not spread by everyday contact, such as hugging or sharing crockery. The virus enters the bloodstream and infects cells that have a particular receptor, known as the CD4 receptor, on their surface. These cells include a type of white blood cell (a CD4 lymphocyte) responsible for fighting infection and cells in other tissues such as the brain. The virus reproduces within the infected cells, which then die, releasing more virus particles into the blood. If the infection is left untreated, the number of CD4 lymphocytes falls, resulting in greater susceptibility to certain infections and some types of cancer. Some people experience a short-lived illness similar to infectious mononucleosis when they are first infected with HIV. Many individuals have no obvious symptoms; some have only vague complaints, such as weight loss, fevers, sweats, or unexplained diarrhoea, described as AIDS-related complex. Minor features of HIV infection include skin disorders such as seborrhoeic dermatitis. More severe features include persistent herpes simplex infections, oral candidiasis (thrush), shingles, tuberculosis, and shigellosis. HIV may also affect the brain, causing a variety of neurological disorders, including dementia. Certain conditions, known as AIDSdefining illnesses, are characteristic of full-blown AIDS. These include cancers ( Kaposi’s sarcoma and lymphoma of the brain), and various infections ( pneumocystis pneumonia, cytomegalovirus infection, toxoplasmosis, diarrhoea due to or, candidiasis, disseminated strongyloidiasis, and cryptococcosis), many of which are described as opportunistic infections. Confirmation of HIV infection involves testing a blood sample for the presence of antibodies to HIV. Diagnosis of fullblown AIDS is based on a positive HIV test along with the presence of an AIDSdefining illness. The risk of infection with HIV can be reduced by practising safer sex. Intravenous drug users should not share needles. There is a small risk to health workers handling infected blood products or needles, but this risk can be minimized by safe practices. Treatment of HIV infection with a combination of antiviral drugs can slow the disease’s progress, and may prevent the development of full-blown AIDS. The 2 main types of antiviral drug used are protease inhibitors, such as indinavir, and reverse transcriptase inhibitors such as zidovudine. Treatment is also available for AIDS-defining illnesses.