All specimens for culture were evaluated by standard microbiological methods. No specimens were processed for viral cultures. The blood culture isolates were considered pathogenic if the organism was known to cause disease in healthy infants. UTI (urinary tract infection) was defined as the isolation of >104 CFU/mL of urine of a single pathogen by catheterization. A positive urine analysis was defined by a positive test for leucocyte esterase or nitrite by the dipstick method or leukocyte ≥10 cells/mm3 in uncentrifuged urine. Methods of viral agent evaluation: in infants with viral respiratory symptoms and complaints (a runny nose, sneezing, coughing), the presence of RSV-Ag (rapid immunochromatographic test produced by Prima lab SA, Switzerland), and influenza-Ag (Immunoassay test produced by Dalian Rongbang Medical Healthy Devices, Spain) were assessed from nasopharyngeal secretions. The body weight of the subjects was taken on admission and weight loss was assessed according to the birth weight. The proportion of weight loss more than twelve percent of birth weight was considered as dehydration.