Level 1: Causes of diarrhea in nine-month-old babies 1. Infectious causes Nine-month-old babies have weak immunity and are easily infected with pathogenic microorganisms such as bacteria and viruses, which can lead to diarrhea. Common infectious causes include bacterial enteritis, viral enteritis, etc. 2. Dietary reasons Nine-month-old babies begin to gradually add complementary foods, but if too much or inappropriate food is given, it can also cause diarrhea. Compare