HIV, or human immunodeficiency virus, is a retrovirus that primarily targets cells involved in immune defence, especially CD4 T lymphocytes. Infection is chronic because the virus integrates its genetic material into host-cell DNA and establishes reservoirs that are not eliminated by currently available standard treatment. HIV is principally transmitted through exposure to infected blood, semen, vaginal fluids or breast milk, with sexual transmission and sharing contaminated injecting equipment being major routes globally. Transmission from parent to child can occur during pregnancy, birth or breastfeeding, but effective medical interventions can greatly reduce perinatal transmission.
HIV belongs to the genus Lentivirus within the Retroviridae family and has two principal types, HIV-1 and HIV-2. HIV-1 accounts for most infections worldwide, whereas HIV-2 is concentrated particularly in West Africa and generally has a slower course and lower transmissibility. The virus uses its envelope proteins to attach to host-cell receptors, including CD4 and a co-receptor such as CCR5 or CXCR4, allowing entry into susceptible cells. Reverse transcriptase converts viral RNA into DNA, integrase inserts viral DNA into the host genome, and other viral processes produce new infectious particles. These mechanisms explain why combinations of drugs targeting different stages of replication are used.
Acute HIV infection may develop approximately weeks after exposure and can resemble a nonspecific viral illness, with fever, rash, sore throat, lymph-node enlargement, fatigue or other symptoms, although some people have no recognisable acute illness. During the subsequent chronic phase, viral replication and immune damage can continue even when the person feels well. Without effective antiretroviral therapy, progressive depletion and dysfunction of CD4 cells can impair immune responses and increase susceptibility to opportunistic infections and certain cancers. The pace of progression varies between individuals and is strongly altered by treatment.
Diagnosis uses laboratory testing designed to detect HIV infection rather than relying on symptoms. Modern screening commonly combines detection of HIV antibodies with the viral p24 antigen, while nucleic-acid testing can detect viral genetic material and may be useful in particular clinical circumstances. Tests differ in their window periods, so a test performed very soon after exposure may not exclude infection. Confirmatory testing is used when screening produces a reactive result. Because diagnosis has substantial medical and social implications, testing services generally incorporate appropriate counselling, confidentiality and linkage to care.
Antiretroviral therapy, usually consisting of combinations of medicines from different drug classes, suppresses HIV replication to very low levels. Treatment does not normally remove latent virus from the body, so therapy is generally continued long term. Viral suppression allows immune recovery, greatly reduces morbidity and mortality, and prevents sexual transmission when an individual maintains an undetectable viral load, commonly summarised by the principle that undetectable HIV is untransmittable sexually. Treatment also reduces the risk of transmission during pregnancy and other exposure circumstances when appropriate clinical measures are followed.
Prevention includes condoms, sterile injecting equipment, HIV testing, treatment of people living with HIV, pre-exposure prophylaxis and post-exposure prophylaxis. Pre-exposure prophylaxis involves antiretroviral medicines taken by people without HIV who have an ongoing risk of acquiring it, while post-exposure prophylaxis is a short course started promptly after a potentially significant exposure. Prevention strategies need to account for the route, timing and circumstances of exposure. HIV cannot be transmitted through ordinary casual contact such as hugging, sharing household utensils or sitting near someone, and it is not spread by insects.
HIV and AIDS are related but are not synonymous. HIV is the infection caused by the virus, whereas AIDS, or acquired immunodeficiency syndrome, describes advanced HIV-related immune deficiency characterised by specific opportunistic illnesses or severe CD4 depletion according to established clinical criteria. Effective treatment means many people with HIV never develop AIDS. A diagnosis of HIV therefore does not mean that a person has AIDS or that progression to AIDS is inevitable. Contemporary antiretroviral therapy has transformed HIV from an illness that commonly caused severe progressive immunodeficiency into a manageable chronic infection for people who can access and adhere to effective treatment.
Major challenges surrounding HIV include late diagnosis, unequal access to treatment, drug resistance, stigma and social barriers to prevention and healthcare. Resistance can arise when viral replication occurs in the presence of inadequate drug pressure, which is one reason treatment regimens are selected carefully and adherence is important. HIV research continues to investigate vaccines, long-acting therapies, remission strategies and approaches to eliminating or controlling viral reservoirs. The history of HIV has also influenced modern infectious-disease science through advances in molecular virology, antiretroviral pharmacology, public-health surveillance and prevention.
