Why does my baby have diarrhea?

It may be normal: Breastfed babies tend to have soft, yellow, mushy stools. Some breastfed babies' stools may appear like diarrhea. This may be entirely normal, especially if your baby is gaining weight well. Make sure your baby is making plenty of wet diapers with urine as that is a good sign of hydration. If in doubt, check with your pediatrician to be sure. Fever, blood or mucus in the stool would be abnormal.

Great question: Many answers, first keep up with fluid loses, also see a pediatrician who can do an evaluation for possible causes. It can be a sugar enzyme deficiency, it can be a intestinal bug or many other possible causes. Treating the symptoms alone wont get the answer, so, some evaluation of the stool may be necessary.