Lactose intolerance in infants is mostly due to lactose intolerance secondary to post-viral infections, with a small portion being primary lactose intolerance caused by low activity or lack of lactase. This leads to undigested lactose after consuming animal milk, resulting in a series of clinical symptoms. Common symptoms include changes in stool; for lactose-intolerant children, there are relatively more curds in the stool, and the stool is somewhat loose and poorly digested. Additionally, children with lactose intolerance often have poor digestive function, increased gastrointestinal gas, and sometimes experience bloating and abdominal discomfort, which can also affect the baby's sleep.