Having fever of 38 degree or above for more than three to four weeks with no clear diagnosis regardless of thorough investigation inpatiently or outpatiently is defined as FUO (Fever of Unknown Origin).

 

This condition is mostly due to connective tissue diseases, malignancies and infections. Studies in Holand showed the main cause of this condition remains mostly undiagnosed (50%) however infections only play a role in 16% of cases.