1. Causes of baby diarrhea There are many reasons for baby diarrhea. It may be infection with viruses or bacteria, or it may be food allergies or indigestion. In addition, if the baby is teething, it may also cause diarrhea. 2. How to judge whether your baby has diarrhea. Normally, the number of bowel movements per day will vary depending on age and diet. But if your baby suddenly has frequent bowel movements,