Fever of Unknown Origin / Pyrexia of unknown origin

Fever of unknown origin (FUO), also known as pyreia of unknown origin (PUO), is a condition in which fever in excess of 38.3C has been around for three weeks or longer, and persists despite three outpatient evaluations or 3 days in the hospital. Despite extensive investigations and initial treatment, fever persists and as the name suggests, the cause of the febrile illness is often not identified during the initial workup. 

FUO / PUO can occur in otherwise healthy individuals, hospitalized patients, immunocompromised patients or patients with human immunodeficiency virus.