The single most common cause of pyrexia of unknown origin is
Tuberculosis is an ancient disease, infection is common in India. So it is the most common cause of Pyrexia of Unknown origin, compared to others. Ref: Baveja textbook of microbiology; 4th edition
Mycobacterium tuberculosis
Salmonella typhi
Brucella species
Salmonella paratyphi A
Bacteriology