We don't know why some people are lactose intolerant and others aren't, but it's not rare. Between 30 and 50 million people in the United States are lactose intolerant. Genetics play a role: About 90 percent of Asian Americans – and as many as 75 percent of African American, Hispanic American, Jewish, and Native American adults – are lactose intolerant. About 15 percent of people of northern European descent have the condition.

Very rarely, a baby is born with lactose intolerance. (Both parents would have to pass the gene for this type of lactose intolerance to the baby.) From birth, the baby would have severe diarrhea and be unable to tolerate the lactose in his mother's breast milk or in formula made from cow's milk. He'd need a special, lactose-free infant formula.

If your baby has had a severe case of diarrhea, his body may temporarily have trouble producing lactase, and he may have symptoms of lactose intolerance for a week or two. Some medications can also cause the body to produce lower levels of lactase, causing temporary lactose intolerance. People with long-term conditions that affect the intestines (like celiac disease or Crohn's disease) sometimes suffer from lactose intolerance as well.