1. Causes of diarrhea in five-month-old babies There may be many reasons for diarrhea in five-month-old babies, such as infectious diarrhea, food allergies, indigestion, etc. Among them, the most common is infectious diarrhea, usually caused by bacteria, viruses or parasites. In addition, if your baby eats food or drinks that are not suitable for him, it may also cause diarrhea. 2. Symptoms of diarrhea in five-month-old babies Symptoms of diarrhea in five-month-old babies include: diarrhea, abdominal bloating, nausea and vomiting