1. What is diarrhea in a one and a half year old baby? A one-and-a-half-year-old baby\’s immunity is not yet fully developed and his digestion is not mature enough, so he is susceptible to invasion by various bacteria and viruses, leading to diarrhea. Diarrhea refers to an increase in the number of bowel movements per day, a decrease in stool volume, and a thinner texture. 2. What are the symptoms of diarrhea in a one and a half year old baby? 1. Diarrhea: The number of bowel movements per day increases, the amount of stool decreases, and the texture becomes thinner. 2. Fever: As the body’s resistance to bacteria and viruses decreases, it is easy to cause fever. 3. Vomiting: Some babies will vomit. 4. Dehydration: Loss of water due to diarrhea