AIDS is the abbreviation for acquired immune deficiency syndrome, a deficiency of the immune system due to infection with the human immunodeficiency virus (HIV). The interval between infection and the development of AIDS is highly variable. Without treatment, around half of those individuals infected will develop AIDS within eight to nine years. In about one in ten cases, however, progression to AIDS is very slow, taking up to 20 years or longer. Illness and death from AIDS is a growing health problem worldwide, and there is, as yet, no cure or vaccine.

Methods of transmission 

HIV is transmitted in body fluids such as semen, blood, vaginal secretions, and breast milk. 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, kidney transplants, and artificial insemination by donated semen; but improved screening has greatly reduced these risks. HIV is not spread by everyday contact, such as hugging or sharing crockery.

Effects of the virus

The virus enters the bloodstream and infects cells with a particular receptor, called the CD4 receptor, on their surface. These cells include a type of white blood cell called a CD4 lymphocyte (a T lymphocyte with a CD4 receptor), that is 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.

Symptoms and signs 

Some people experience a short-lived illness similar to infectious mononucleosis when they are first infected with HIV. Many individuals have no obvious symptoms. After the initial illness, many people remain well. Some may suffer from enlarged lymph nodes, muscle pain, and excessive sweating. Severe bacterial infections, such as pneumonia, are common. Later, vague complaints, such as weight loss, fevers, sweats, or unexplained diarrhoea (described as AIDS-related complex) may herald the development of AIDS.

Other features of infection with HIV include skin disorders and a variety of viral, fungal and bacterial infections. HIV may also affect the brain, causing neurological disorders such as dementia. Certain conditions, known as AIDS defining illnesses, mark the development of full-blown AIDS. These include cancers (lymphoma of the brain, Kaposi’s sarcoma, and cancer of the cervix) and various infections (pneumocystis pneumonia, cytomegalovirus infection, toxoplasmosis, diarrhoea as a result of cryptosporidium or isospora, 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 (see HIV test), which may not develop for three months after initial infection. The condition is monitored using blood tests that measure the number of CD4 lymphocytes in the blood or by measuring viral load (the amount of virus detectable in the blood). Diagnosis of full-blown AIDS is based on a positive HIV test along with the presence of an AIDS-defining illness.

Treatment and outlook 

Treatment of HIV infection with a combination of antiviral drugs can slow the progress of the disease, and may prevent the development of full-blown AIDS. The main types of antiviral drug used are protease inhibitors, such as indinavir, and reverse transcriptase inhibitors such as zidovudine. Several drugs are usually used together to prevent resistance from developing. AIDS-defining illnesses are treated as they develop.

Since the introduction of antiviral drug combination therapies, deaths from AIDS in the developed world have been reduced dramatically. HIV infection remains life-threatening, however, and the most effective strategy for defeating it is prevention of infection.

Prevention of infection 

The risk of infection can be reduced by practising safer sex. and by intravenous drug users not sharing needles. There is a small risk to health workers handling infected needles or blood products, but this can be minimized by the adoption of safe practices in the workplace.