What is the cause of frequent fevers?

Q: My boyfriend gets fevers once in every few weeks, usually at night. The fever ranges from 102.5 Fahrenheit and above. He has no idea what causes them. He is a plumber and has to work outside often. However, it occurs on not so hot days as well, in spite of plenty of sleep, liquids, food and breaks. Please advise.

A:A pyrexia (fever) of unknown origin (PUO) is defined by the following criteria– a) a temperature greater than 38.3°C (101°F) on several occasions; b) more than 3 weeks duration of illness and c) a failure to reach a diagnosis despite 1 week of inpatient investigation. The most common causes are infections (TB, urinary tract infections, hepato-biliary infection, osteomyelitis, endocarditis, viral or parasitic infections), neoplasms (lymphomas, leukaemias, solid tumours), collagen vascular diseases (SLE, systemic onset JRA, polyarteritis nodosa, rheumatoid arthritis, mixed connective tissue disease) and many miscellaneous disorders (granulomatous disorders, drug fever, endocrine disorders etc). No diagnosis can be made in nearly 5-15% of patients despite exhaustive investigations. As is evident, the list of causes is large and an exhaustive history combined with a thorough physical examination (which may need to be repeated several times) is required. This usually gives an idea of the underlying disorder so that appropriate investigations can be done.