AIDS, or acquired immunodeficiency syndrome, is the advanced clinical stage of infection with HIV in which substantial immune dysfunction has developed. The term does not describe a separate virus and is not synonymous with HIV itself. Diagnostic definitions vary somewhat by jurisdiction and clinical framework, but AIDS generally involves a severely impaired immune system identified through a sufficiently low CD4 count or the occurrence of particular AIDS-defining conditions in a person with HIV. Opportunistic infections and certain malignancies become more likely because immune surveillance and cellular defence have been compromised.
The biological basis of AIDS is progressive HIV-mediated damage to the immune system. HIV preferentially infects CD4 T cells and can also affect other immune cells and tissues. Persistent viral replication, immune activation and direct or indirect cellular damage progressively reduce immune competence when treatment does not adequately suppress the virus. As CD4-cell function and number decline, organisms that are normally controlled by an intact immune system can cause serious disease. The syndrome therefore represents the consequences of advanced HIV-related immunodeficiency rather than a single pathological process or single infectious organism.
AIDS-defining illnesses include particular opportunistic infections and malignancies whose occurrence in a person with HIV indicates advanced immunodeficiency. Examples include Pneumocystis jirovecii pneumonia, certain forms of tuberculosis, cryptococcosis, toxoplasmosis of the brain, oesophageal candidiasis, cytomegalovirus disease and Kaposi sarcoma, among others recognised in clinical definitions. These conditions can be severe because the immune response required to contain them is impaired. Their presence may sometimes lead to the diagnosis of advanced HIV infection in someone who did not previously know they had HIV.
Before effective combination antiretroviral therapy became widely available, AIDS commonly progressed through severe opportunistic disease, wasting, neurological complications and malignancies and was associated with high mortality. The introduction and expansion of effective antiretroviral treatment fundamentally changed this trajectory. Modern therapy can suppress HIV replication, permit partial or substantial immune recovery and prevent many people with HIV from developing AIDS. People who have already developed AIDS can also improve substantially after treatment and appropriate management of opportunistic illnesses, although some immune damage or organ complications may persist.
Diagnosis involves establishing HIV infection and determining whether recognised criteria for AIDS have been met. CD4 measurements provide information about immune status, while clinical assessment identifies opportunistic infections, cancers and other complications. Viral load measures HIV replication and is central to treatment monitoring, although viral load alone does not define AIDS. Clinicians may also investigate organ function, nutritional status, co-infections and treatment complications because advanced immunodeficiency can involve multiple systems simultaneously.
Management requires antiretroviral therapy together with diagnosis and treatment or prevention of opportunistic diseases. Depending on immune status and clinical circumstances, preventive antimicrobial treatment may be used against particular infections, while suspected opportunistic illnesses require targeted therapy. Drug interactions are clinically important because antiretroviral medicines and antimicrobial agents can affect the metabolism or toxicity of one another. Restoring immune function with HIV treatment is a central objective, but clinicians may need to manage immune reconstitution inflammatory syndrome, in which recovering immune responses can temporarily produce intense inflammation against an existing infection.
A major misconception is that an HIV diagnosis automatically means AIDS. In modern clinical practice, effective treatment often prevents progression to AIDS, and a person previously diagnosed with AIDS may experience immune recovery even though the historical diagnosis remains clinically relevant. Another misconception is that AIDS itself is transmissible. It is not a separate transmissible agent; HIV is the transmissible infection. Transmission occurs through specific exposure routes rather than casual contact. Understanding this distinction is important for accurate health information and for avoiding stigma toward people living with HIV or AIDS.
AIDS remains important as a marker of advanced HIV disease, particularly where diagnosis or treatment is delayed. Its frequency is strongly influenced by access to testing, antiretroviral therapy, retention in care and prevention programmes. The continuing existence of AIDS demonstrates the consequences of untreated or inadequately treated HIV, while the decline in AIDS-related illness in settings with broad treatment access demonstrates the effectiveness of modern antiretroviral therapy. Public-health efforts therefore focus heavily on early diagnosis, rapid linkage to care, sustained viral suppression and prevention of new HIV infections.
