1. Causes of diarrhea in four-month-old infants 1.1 Dietary problems 1.2 Underdeveloped digestion 1.3 Virus or bacterial infection Diarrhea means that the number of bowel movements in infants increases and the feces becomes thin and even watery. In babies as young as four months, diarrhea can be caused by dietary problems, underdeveloped digestion, and viral or bacterial infections. 1. Dietary problems At four months of age, the baby begins to gradually add complementary foods. If too much complementary food is added or is too irritating, it can easily lead to indigestion and diarrhea in the baby. Additionally, if breast milk or formula is of poor quality or stored improperly, it may