Fever of unknown origin (FUO) is a higher body temperature with no clear cause, even though there has been at least one or two weeks of testing.

The cause of this fever is unknown. In some people the cause may never be known.

Factors that may make it difficult to find a cause include:

  • A common illness that does not have the usual symptoms
  • Illness with other symptoms that may appear later
  • Illnesses that may have a delayed positive test
  • Person is unable to communicate about other symptoms such as an infant or someone in a coma
  • Genetic condition that causes periodic fevers—rare