HIV

HIV, the human immunodeficiency virus, is a retrovirus of the lentivirus group. Its genome is two copies of single-stranded RNA packed in a cone-shaped capsid with three enzymes, reverse transcriptase, integrase and protease, all wrapped in a lipid envelope. Envelope proteins bind CD4 and a coreceptor, CCR5 or CXCR4, on helper T cells and macrophages. Inside the cell, reverse transcriptase copies the RNA into DNA, and integrase splices that DNA into a host chromosome, where it can hide for years. HIV-1 causes most infections worldwide, while HIV-2 is found mainly in West Africa.

The virus spreads through blood, semen, vaginal fluids and breast milk, chiefly by sex without condoms, shared needles and from mother to child, but not by everyday contact. A few weeks after infection many people have a flu-like illness, and a long silent phase follows while CD4 T cells steadily decline. When the count falls below 200 cells per cubic millimeter, or certain opportunistic infections or cancers appear, the person has AIDS, acquired immunodeficiency syndrome.

HIV is diagnosed with tests for antibodies, viral antigen or viral RNA. There is no cure, but antiretroviral therapy, a combination of drugs aimed at different viral steps, suppresses the virus so people can live long lives, and those with an undetectable viral load do not pass HIV on through sex.