Fever of Unknown Origin: Fever of unknown origin (FUO) is a fever greater than 38.3°C (101 °F) that has occurred intermittently for over three weeks, for which no specific cause readily identified.

In 85 to 95% of cases, the specific cause for a fever of unknown origin can eventually be identified after extensive diagnostic testing. The causes of a fever of unknown origin that are eventually discovered typically fall into one of three categories: uncommon infections, cancers, and chronic inflammatory diseases (most commonly connective tissue diseases or autoimmune diseases).