Most pyrexias result from a clearly defined illness, e.g. acute pyelonephritis or acute appendicitis, or from self-limiting viral infections, e.g. common cold. Pyrexia of unknown origin is defined as a temperature>38.3°C on several occasions, accompanied by more than three weeks of illness and failure to reach a diagnosis after one week of inpatient investigation. Most cases of pyrexia of unknown origin are unusual presentations of common diseases, e.g. tuberculosis, endocarditis, rather than rare or exotic illnesses.