What is HIV?

HIV (human immunodeficiency virus) is a retrovirus that infects immune cells, particularly CD4 cells (or T-lymphocytes). If left untreated, it causes a decline in immunity that leads to opportunistic infections, malignancies and other morbidities, eventually resulting in death.

When someone has HIV infection, the virus will be present in their blood, semen, vaginal secretions, rectal secretions and breast milk. HIV can be transmitted through sexual activity or sharing of drug injecting equipment, and also from mother to child before, during or after delivery. In most countries, blood products are heat-treated and blood for transfusion is routinely screened to avoid the risk of transmitting HIV and other blood-borne viruses.

HIV virions (Dr. A. Harrison; Dr. P. Feorino(CDC))

Natural history of HIV

Stages of HIV infection

Maculopapular rash of primary HIV infection (Henry de Vries)


In the first few weeks following infection, antibodies to HIV are produced (‘seroconversion’). This early stage is called ‘primary HIV infection’ and often causes a flu-like illness with a rash, and most often an infectious mononucleosis-like condition with a sore throat, although some people have no symptoms. The amount of virus in the blood (‘viral load’) spikes at this early stage and the risk of transmitting HIV is very high. (Care is needed when testing patients who may have primary HIV infection, as an HIV antibody test can give a false negative result – see Types of HIV test.)

Asymptomatic and symptomatic HIV infection

Once antibodies have developed, the amount of virus declines and someone with HIV may have no signs or symptoms of HIV for months or even years. However, the CD4 cells are gradually declining and the viral load is rising again, leading to the development of conditions associated with reduced immunity. These include a number of skin conditions such as herpes zoster, recurrent herpes simplex, and recurrent or severe seborrheic dermatitis, as well as opportunistic infections and certain malignancies.

Seborreic eczema in a patient with HIV (Henry de Vries)

AIDS (acquired immune deficiency syndrome).

This is a term used to describe HIV disease when certain specified conditions are present or the CD4 count has fallen to below 200 cells/mm3.

The graph shows the course of HIV infection and how the viral load and CD4 count interact.

Why test for HIV? / HIV epidemiology in Europe

Continue >