1. What is diarrhea in nine-month-old babies? Diarrhea in nine-month-old babies refers to frequent diarrhea in nine-month-old babies. Usually, the stool frequency is more than 3 times a day, and the stool is watery or muddy, accompanied by abdominal bloating, abdominal pain and other uncomfortable symptoms. 2. What are the causes of diarrhea in nine-month-old babies? 1. Infection: Microbial infections such as bacteria, viruses and parasites are the main causes of diarrhea in infants and young children. 2. Improper diet: For example, eating too much food containing preservatives, pigments and other chemical substances, or adding too much salt and MSG, etc.