1. Causes of diarrhea in babies three and a half months old 1.1 Dietary problems 1.2 Infectious bacteria 1.3 Side effects of drugs It is very common for babies three and a half months old to have diarrhea. This may be caused by dietary problems, bacterial infections or side effects of drugs. First, if babies are eating solid foods, they may be allergic to or unable to digest some new foods, which can lead to diarrhea. Secondly, if the baby has a bacterial infection or