In the absence of a microbiological lab testing, majority of the cases of febrile illnesses in Nepal are diagnosed on a clinical basis alone. These cases of undifferentiated febrile illness are treated empirically with multiple antibiotics without an etiological diagnosis. Unnecessary use of antibiotics can increase the risk of antibiotic resistance and adds unnecessary burden of cost to the patients and their families. This study aims to determine the etiology of infections using additional microbiological techniques in patients admitted to the hospital with acute undifferentiated febrile illness of 3 days to 21 days duration, and assess its impact on use of antimicrobial drugs. In addition to the clinical features, the etiological diagnosis will be confirmed by using additional tests including culture techniques and ELISA based serological tests for 7 additional pathogens which are currently not tested in Nepal. Patients meeting definition of undifferentiated fever of 3 days to 21 days duration will be enrolled from a major tertiary care center using predefined inclusion and exclusion criteria.This study will help to identify additional infectious diseases prevalent in Nepal, decrease unnecessary empirical use of multiple antibiotics, and improve patient outcome. Additionally, identification of new causes of febrile illness will help in prevention and control of these infectious diseases at public health level.