• Crohn's disease and ulcerative colitis (collectively known as inflammatory bowel diseases)

Affect: 1.5 million Americans, including about 150,000 children.

Symptoms: Diarrhea, abdominal cramps, weight loss, bleeding, low-grade fever, painful joints. In children, may cause lack of growth and sexual development.

• Irritable bowel syndrome

Affects: As many as 20 percent of Americans, and more women than men.

Symptoms: Cramping, abdominal pain, bloating, constipation and diarrhea.

• Lactose intolerance

Affects: 30 million to 50 million Americans. Most African-Americans, American Indians and Asian-Americans are lactose intolerant.

Symptoms: Nausea, cramps, bloating, gas and diarrhea, for 30 minutes to two hours after eating or drinking foods containing lactose.

—T. DeAngelis