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Initially in 1990, a four-stage clinical staging system was developed for clinical purposes and only for adults. Subsequently in 2002, a three-stage system for children was proposed to support rolling out ART. This publication revises the 2003 WHO clinical staging of HIVrelated disease in infants and children, which is now harmonized with the 1990 classification of disease for adults and adolescents. This is similar to the four-stage clinical classification of the United States CDC revised in 1994 and originally intended for surveillance purposes. Both the United States CDC and WHO clinical classifications recognize primary HIV infection. It is also proposed that the appearance of new or recurrent clinical staging events or immunodeficiency be used to assess individuals once they are receiving ART. Clinical assessment prior to treatment Clinical staging is used once HIV infection has been confirmed (serological and/or virological evidence of HIV infection). An additional presumptive clinical diagnosis of severe HIV disease (equivalent to severe immunodeficiency) among infants younger than 18 months is suggested for use in situations in which definitive virological diagnosis of HIV infection is not readily available. The clinical events used to categorize HIV disease among infants, children, adolescents or adults living with HIV are divided into those for which a presumptive clinical diagnosis may be made (where syndromes or conditions can be diagnosed clinically or with basic ancillary investigations) and those requiring a definitive diagnosis (generally conditions described according to causation requiring more complex or sophisticated laboratory confirmation). We will provide the clinical stage in simplified terms describing the spectrum of HIV related symptomatology, asymptomatic, mild symptoms, advanced symptoms and severe symptoms. The clinical staging events, and provides further details of the specific events and the criteria for recognizing them. The clinical stage is useful for assessment at baseline (first diagnosis of HIV infection) or entry into long-term HIV care and in the follow-up of patients in care and treatment programmes. It should be used to guide decisions on when to start co-trimoxazole prophylaxis and other HIV related interventions, including when to start antiretroviral therapy. The clinical stages have been shown to be related to survival, prognosis and progression of clinical disease without antiretroviral therapy in adults and children.