Fever of unknown origin (FUO), also known as pyrexia of unknown origin, is commonly defined according to the Petersdorf criteria as fever ≥38.3° (101 Fahrenheit) lasting ≥3 weeks that remains undiagnosed after 1 week of investigations [1]. The last criterion has recently undergone modification, and is now generally interpreted as no diagnosis after appropriate inpatient or outpatient evaluation [2]. Causes of FUO include more than 200 diseases [3], but the three major pathological categories are represented by infections, malignancies, and non-infectious inflammatory diseases.
