While a severe case of diarrhea can alarm even the most unflappable parents, rest assured that in the United States most cases of diarrhea are relatively mild and don't pose a major health threat as long as your baby doesn't get dehydrated. But dehydration can be very serious, even fatal, in a baby, so it's crucial that you make sure your baby is getting plenty of liquids. What causes diarrhea? Here is a long list of bacterial infection. It might also be the result of a parasite, a course of antibiotics, or something he ate. Viral infection Bacterial infection Ear infection Parasites Antibiotics Too much juice Food allergy Milk protein Food intolerance Lactose intolerance Poisoning