Probiotics are organisms such as bacteria or yeast that are believed to improve health. They are available in supplements and foods. The idea of taking live bacteria or yeast may seem strange at first. After all, we take antibiotics to fight bacteria. But our bodies naturally teem with such organisms. The digestive system is home to more than 500 different types of bacteria. They help keep the intestines healthy and assist in digesting food. They are also believed to help the immune system.

How Do Probiotics Work?
Researchers believe that some digestive disorders happen when the balance of friendly bacteria in the intestines becomes disturbed. This can happen after an infection or after taking antibiotics. Intestinal problems can also arise when the lining of the intestines is damaged. Taking probiotics may help. "Probiotics can improve intestinal function and maintain the integrity of the lining of the intestines," says Stefano Guandalini, MD, professor of pediatrics and gastroenterology at the University of Chicago Medical Center. These friendly organisms may also help fight bacteria that cause diarrhea.

Probiotics and the Immune System
There's also evidence that probiotics help maintain a strong immune system. "In societies with very good hygiene, we've seen a sharp increase in autoimmune and allergic diseases," Guandalini tells WebMD. "That may be because the immune system isn't being properly challenged by pathogenic organisms. Introducing friendly bacteria in the form of probiotics is believed to challenge the immune system in healthy ways."

Cautions About Probiotics
For the most part, taking probiotics is safe and causes few side effects. "People in cultures around the world have been eating yogurt, cheeses, and other foods containing live cultures for centuries," says Martin Floch, MD, a professor of gastroenterology at Yale University, co-author of Probiotics: A Clinical Guide, and a consultant for the Dannon Company.

Still, probiotics may be dangerous for people with weakened immune systems or serious illnesses. One study found that patients with severe pancreatitis who were given probiotics had a higher risk of death.