Crohn's disease is an illness that causes the intestines, or other parts of the digestive tract (the parts of the body food goes through), to become swollen, and sometimes develop ulcers (holes). People with Crohn's disease often have pain in the gut, diarrhea, vomiting, and weight loss. Crohn's can also cause skin rashes, arthritis, and swollen eyes.
Nobody knows exactly what causes Crohn's disease. In the disease, the person's body attacks itself.