Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Crohn's Disease

What Is It?

Crohn's disease is one of the inflammatory bowel diseases. It is a long-term (chronic) condition in which inflammation causes injury to the intestines. Crohn's disease typically begins in young adulthood, most often between ages 15 and 40.

No one knows for sure what triggers the initial intestinal inflammation at the start of Crohn's disease. Many experts think that a virus or a bacterial infection might start the process by activating the immune system, and that the body's immune system stays active and creates inflammation in the intestine even after the infection goes away. Family members may share genes that make Crohn's disease more likely to develop if the right trigger occurs. Ten percent to 15 percent of people who have Crohn's disease have at least one relative with Crohn's disease or other type of inflammatory bowel disease (such as ulcerative colitis). Crohn's disease is more common in people of Jewish heritage, relative to non-Jews.

Once Crohn's disease begins, it can cause lifelong symptoms that come and go. The inside lining and deeper layers of the intestine wall become inflamed. The lining of the intestine becomes irritated, and can thicken or wear away in spots. This creates ulcers, cracks and fissures. Inflammation can allow an abscess (a pocket of pus) to develop.

A unique complication of Crohn's disease is called a fistula. A fistula is an abnormal connection between organs in the digestive tract, usually a connection between one piece of the intestine and another. A fistula can be created after inflammation becomes severe. To understand how a fistula is created, consider the way the intestine attempts to heal. Between attacks of inflammation, the intestine recoats itself with a new lining. When the inflammation has been severe, the intestine can lose its ability to distinguish the inside of one piece of intestine from the outside of another piece. As a result, it can mistakenly build a lining along the edges of an ulcer that has worn through the whole wall of the intestine, forming a fistula.

The section of the small intestine called the ileum (in the right lower abdomen) is especially prone to damage from Crohn's disease. However, ulcers and inflammation can occur in all areas of the digestive tract, from the mouth all the way to the rectum. A few other parts of the body, such as the eyes and joints, also can be affected in people with Crohn's disease.

Crohn's disease is one of the inflammatory bowel diseases. It is a long-term (chronic) condition in which inflammation causes injury to the intestines. Crohn's disease typically begins in young adulthood, most often between ages 15 and 40.

No one knows for sure what triggers the initial intestinal inflammation at the start of Crohn's disease. Many experts think that a virus or a bacterial infection might start the process by activating the immune system, and that the body's immune system stays active and creates inflammation in the intestine even after the infection goes away. Family members may share genes that make Crohn's disease more likely to develop if the right trigger occurs. Ten percent to 15 percent of people who have Crohn's disease have at least one relative with Crohn's disease or other type of inflammatory bowel disease (such as ulcerative colitis). Crohn's disease is more common in people of Jewish heritage, relative to non-Jews.

Once Crohn's disease begins, it can cause lifelong symptoms that come and go. The inside lining and deeper layers of the intestine wall become inflamed. The lining of the intestine becomes irritated, and can thicken or wear away in spots. This creates ulcers, cracks and fissures. Inflammation can allow an abscess (a pocket of pus) to develop.

A unique complication of Crohn's disease is called a fistula. A fistula is an abnormal connection between organs in the digestive tract, usually a connection between one piece of the intestine and another. A fistula can be created after inflammation becomes severe. To understand how a fistula is created, consider the way the intestine attempts to heal. Between attacks of inflammation, the intestine recoats itself with a new lining. When the inflammation has been severe, the intestine can lose its ability to distinguish the inside of one piece of intestine from the outside of another piece. As a result, it can mistakenly build a lining along the edges of an ulcer that has worn through the whole wall of the intestine, forming a fistula.

The section of the small intestine called the ileum (in the right lower abdomen) is especially prone to damage from Crohn's disease. However, ulcers and inflammation can occur in all areas of the digestive tract, from the mouth all the way to the rectum. A few other parts of the body, such as the eyes and joints, also can be affected in people with Crohn's disease.