The single most common cause of pyrexia of unknown origin is
Mycobacterium tuberculosis
Salmonella typhi
Brucella species
Salmonella paratyphi A
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
Bacteriology