Overview
Fever of unknown origin (FUO) in adults is defined as a temperature higher than 38.3°C (101°F) that lasts for more than three weeks with no obvious source despite appropriate investigation. Even when a doctor can't determine the cause of the fever at first, a diagnosis is a step toward treating it.
Fever is the body's normal response to infection and other conditions, both minor and serious. Most fevers help the body fight infection.
The average normal body temperature is 98.6°F (37°C) orally. It can change during the day from a low of 97.4°F (36.3°C) in the morning to a high of 99.6°F (37.6°C) in the late afternoon. Mild elevations—( 99.8°F (37.7°C) — can be caused by too much clothing, a hot bath, or hot weather.
Fever in a child is present when a child's temperature is 100.4°F (38°C) or higher measured rectally.