Many of the body’s immune cells cluster around the intestines, says Denning. Most of the time, those immune cells ignore the trillions of healthy bacteria that live in the gut. “But for some people, that tolerance seems to be broken,” says Denning, “and their immune cells begin to react to the bacteria, creating chronic inflammation.”
The immune cells can attack the digestive tract itself, an autoimmune condition known as inflammatory bowel disease (IBD), which includes ulcerative colitis and Crohn’s disease. The symptoms include diarrhea, cramps, ulcers, and may even require surgical removal of the intestines. Doctors aren’t exactly sure why some people get IBD, but genetics, environment, antibiotics, diet, and stress management all seem to play a role.