What it is: Celiac disease is an intolerance to gluten, a protein found in wheat, rye and barley. When people with celiac disease eat gluten-containing foods, their immune system attacks and damages the lining of the small intestine.

Number of children affected: Approximately 1 in 133 Americans has celiac disease. Children whose parents, siblings, aunts or uncles have celiac disease are most likely to have the condition.

What it is: It’s perfectly normal for babies to spit up many times per day in the first few months of life. But if your baby is also irritable and has difficulty eating, your child might have GERD, a condition in which stomach acid backs up and irritates the esophagus, the tube leading from the mouth to the stomach.

Number of children affected: Two-thirds of four-month-old babies have symptoms of GERD. By age one, about 10% of infants have GERD.

What it is: Lactose is a naturally occurring sugar in milk. People who are lactose-intolerant lack the enzyme needed to break down this sugar, so they can’t digest it. Symptoms include diarrhea, stomach cramps, and gas or bloating after eating dairy products.

Number of children affected: It’s difficult to say. Almost all human babies can digest lactose at birth. By age 20, about 30 million Americans have some degree of lactose intolerance.