The development of complex criteria for the diagnosis, differential diagnosis, and optimization of treatment of infectious diarrhea associated with opportunistic Enterobacteriaceae is a pressing issue of pediatric research and practice. The paper reports a clinical case of protracted intestinal infection associated with Klebsiella pneumoniae in the form of moderate hemorrhagic enterocolitis in an infant, which is explained by the decrease in specific resistance due to unfavorable maternal obstetric and gynecological history, perinatal CNS injury, iron deficiency anemia, protein-energy malnutrition. The disease relapse associated with secondary norovirus infection was reported after the first hospitalization. Three courses of intestinal antiseptics and probiotics were required to achieve a beneficial treatment outcome, although usually in such a situation one course of such drugs is enough. The recovery process was accompanied by the nutritional status improvement, hemorrhagic colitis relief, normalization of gut microbiota.