The single most common cause of pyrexia of unknown origin is -

Pyrexia of unknown origin is definedvas a temperature persistently above 38 degree celsius for more than 3 weeks,without diagnosis despite initial investigations during 3 days of inpatient care or after more than two outpatient visits. Specific organisms associated with PUP are tuberculosis( extrapulmonary - most common cause), HIV-1 infection,other viral infections (cytomegalovirus,EBV), fungal infections (Aspergillus spp.,candida spp., dimorphic fungi), infections with fastidious organisms(Baonella spp., Tropheryma whipplei). Reference Harrison20th edition pg 1232