HIV/AIDS

===Classifications===
Two main clinical staging systems are used to classify HIV and HIV-related disease for [[Disease surveillance|surveillance]] purposes: the [[WHO disease staging system for HIV infection and disease]],<ref name=WHOCase2007/> and the [[CDC classification system for HIV infection]].<ref name=CDCCase2008/> The CDC's classification system is more frequently adopted in developed countries. Since the WHO's staging system does not require laboratory tests, it is suited to the resource-restricted conditions encountered in developing countries, where it can also be used to help guide clinical management. Despite their differences, the two systems allow a comparison for statistical purposes.<ref name=M121/><ref name=WHOCase2007/><ref name=CDCCase2008/>

The World Health Organization first proposed a definition for AIDS in 1986.<ref name=WHOCase2007/> Since then, the WHO classification has been updated and expanded several times, with the most recent version being published in 2007.<ref name=WHOCase2007>{{cite book |title=WHO case definitions of HIV for surveillance and revised clinical staging and immunological classification of HIV-related disease in adults and children |pages=6–16 |url=https://www.who.int/hiv/pub/guidelines/HIVstaging150307.pdf |year=2007 |publisher=World Health Organization |location=Geneva |isbn=978-92-4-159562-9 |url-status=live |archive-url=https://web.archive.org/web/20131031044253/http://www.who.int/hiv/pub/guidelines/HIVstaging150307.pdf |archive-date=October 31, 2013 }}</ref> The WHO system uses the following categories:
* Primary HIV infection: May be either asymptomatic or associated with acute retroviral syndrome<ref name=WHOCase2007/>
* Stage I: HIV infection is [[asymptomatic]] with a CD4<SUP>+</SUP> T cell count (also known as CD4 count) greater than 500 per microlitre (µl or cubic mm) of blood.<ref name=WHOCase2007/> May include generalized lymph node enlargement.<ref name=WHOCase2007/>
* Stage II: Mild symptoms, which may include minor [[Mucous membrane|mucocutaneous]] manifestations and recurrent [[upper respiratory tract infection]]s. A CD4 count of less than 500/µl<ref name=WHOCase2007/>
* Stage III: Advanced symptoms, which may include unexplained [[Chronic (medical)|chronic]] diarrhea for longer than a month, severe bacterial infections including tuberculosis of the lung, and a CD4 count of less than 350/µl<ref name=WHOCase2007/>
* Stage IV or AIDS: severe symptoms, which include [[toxoplasmosis]] of the brain, [[candidiasis]] of the [[esophagus]], [[trachea]], [[bronchi]], or [[lung]]s, and [[Kaposi's sarcoma]]. A CD4 count of less than 200/µl<ref name=WHOCase2007/>

The U.S. Centers for Disease Control and Prevention also created a classification system for HIV, and updated it in 2008 and 2014.<ref name=CDCCase2008>{{cite journal |vauthors=Schneider E, Whitmore S, Glynn KM, Dominguez K, Mitsch A, McKenna MT |title=Revised surveillance case definitions for HIV infection among adults, adolescents, and children aged <18 months and for HIV infection and AIDS among children aged 18 months to <13 years – United States, 2008 |journal=MMWR. Recommendations and Reports |volume=57 |issue=RR-10 |pages=1–12 |date=December 2008 |pmid=19052530 |url=https://www.cdc.gov/mmwr/PDF/rr/rr5710.pdf |access-date=October 17, 2020 |archive-date=October 17, 2020 |archive-url=https://web.archive.org/web/20201017160943/https://www.cdc.gov/mmwr/PDF/rr/rr5710.pdf |url-status=live }}</ref><ref name=CDC2014Clas>{{cite journal |title=Revised surveillance case definition for HIV infection – United States, 2014 |journal=MMWR. Recommendations and Reports |volume=63 |issue=RR-03 |pages=1–10 |date=April 2014 |pmid=24717910 |author1=Centers for Disease Control Prevention (CDC) |url=https://www.cdc.gov/mmwr/pdf/rr/rr6303.pdf |access-date=October 17, 2020 |archive-date=October 17, 2020 |archive-url=https://web.archive.org/web/20201017133412/https://www.cdc.gov/mmwr/pdf/rr/rr6303.pdf |url-status=live }}</ref> This system classifies HIV infections based on CD4 count and clinical symptoms, and describes the infection in five groups.<ref name=CDC2014Clas/> In those greater than six years of age it is:<ref name=CDC2014Clas/>
* Stage 0: the time between a negative or indeterminate HIV test followed less than 180 days by a positive test
* Stage 1: CD4 count ≥ 500&nbsp;cells/µl and no AIDS-defining conditions
* Stage 2: CD4 count 200 to 500&nbsp;cells/µl and no AIDS-defining conditions
* Stage 3: CD4 count ≤ 200&nbsp;cells/µl or AIDS-defining conditions
* Unknown: if insufficient information is available to make any of the above classifications.

For surveillance purposes, the AIDS diagnosis still stands even if, after treatment, the CD4<SUP>+</SUP> T cell count rises to above 200 per µL of blood or other AIDS-defining illnesses are cured.<ref name="M121"/>