No one knows for sure what causes inflammatory bowel disease (IBD). Experts think that abnormal action of a person's immune system may trigger IBD. The immune system is made up of various cells and proteins. Normally, the immune system protects the body from infections caused by viruses or bacteria. Once the infection has cleared up, the immune system "shuts off."

But in people with IBD, the immune system seems to overreact to normal bacteria in the digestive tract. And once it starts working, the immune system fails to "shut off." This causes the inflammation, which damages the digestive tract and causes symptoms.

IBD runs in families. This suggests that inherited factors called genes play a role in causing IBD. Experts think that certain genes may cause the immune system to overreact in IBD.

Stress and eating certain foods do not cause IBD. But both can make IBD symptoms worse.