Level 1: Causes of infant diarrhea 1. Infectious diarrhea Infectious diarrhea is an intestinal infection caused by viruses, bacteria or other microorganisms, which usually leads to diarrhea, vomiting, fever and other symptoms. Babies are more susceptible to infections because their immune systems are not fully developed. 2. Allergic Diarrhea Allergic diarrhea is caused by the baby being allergic to certain foods.