Infectious diseases are important causes of fever of unknown origin (FUO). The spectrum of infectious agents is broad and diagnosis depends on careful evaluation of individual risk factors. Infectious diseases presenting as FUO are frequently atypical presentations of well known infections, e.g. tuberculosis or infectious endocarditis. In this review we present an overview of infectious causes of FUO classified into community acquired infections, nosocomial infections, and infections in immunocompromised hosts.