HIV is the virus responsible for AIDS, the acquired immunodeficiency syndrome that occurred in the early 80s of the twentieth century. HIV is a retrovirus, a group of viruses that uses a particular process, called reverse transcriptase, to replicate itself in the host organism. HIV, according to scientists comes from a "species jump" from ape to humans, was first isolated in 1983, two years after the first known cases of AIDS.

Human immunodeficiency virus (HIV) infection is caused by 1 of 2 similar retroviruses (HIV-1 and HIV-2) that destroy CD4+ T lymphocytes and inhibit cell-mediated immune response, with an increased risk of some infections and some cancers. Primary infection can produce nonspecific feverish syndrome. The risk of subsequent manifestations, correlated with the state of immunodeficiency, is proportional to the level of depletion of the circulating CD4+ lymphocyte.

HIV can cause direct damage to the brain, gonads, kidneys and heart, causing cognitive impairment, hypogonadism, kidney failure and cardiomyopathy. Clinical manifestations include asymptomatic carrier status up to acquired immunodeficiency syndrome, a condition defined by the onset of serious opportunistic infections or tumors or a circulating CD4 count less than 200/ul. HIV infection can be diagnosed by searching for specific Ab, nucleic acids (HIV RNA), or viral Ags (p24). Screening should be routinely offered to both adults and adolescents. Treatment aims to suppress HIV replication by using combinations of many drugs that inhibit HIV enzymes; therapy is able to restore immune function in most patients if suppression of replication is sustained.

Thanks to therapies, AIDS is now a much more manageable disease, at least in more developed countries; while, despite many attempts, an HIV vaccine has not yet been developed.