Background
Fever is one of the most common causes of children’s referral to pediatric emergency department (ED), and almost 20% of General ED referrals relate to febrile children. When it comes to febrile pediatrics patients, co-infection is a very important issue to discuss. This topic has been considered in current literature so, we considered to examine the epidemiology of these co-infections in febrile children younger than 5 years in emergency department.
Materials and Methods: This cross-sectional study was carried out retrospectively with the enrollment of children younger than 5-years-old (accessible sampling) in the emergency department of Mofid Hospital, Tehran, Iran, during November 2017 to December 2018.  After designing and completing a check list.  
Results: Totally, 388 patients with the mean age of 25.4 ± 12.4 months were studied (51.5% Girl).  The source of infection was unclear after using all of the diagnostic tools in 27.5% of cases. Based on the findings fever-associated diarrhea (132 patients, 46.9%), and cellulitis (1 patient, 0.4%) were the most and least frequent source of infections, respectively. The co-incidence of different source of infections showed that the most frequent co-infections were sepsis and UTI; otitis and UTI; pneumonia and UTI; pneumonia and URI; and sepsis and otitis.
Conclusion
It seems that co-infections have been significantly increased in girls and in theage range of 6-36 months. The most and least prevalent infections were fever-associated diarrhea and cellulitis among the patients with at least one type of infection. The co-incidence of different source of infection showed that the most frequent co-infections were sepsis and UTI; otitis and UTI; pneumonia and UTI; pneumonia and URI; and sepsis and otitis.