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.