The exact cause of Crohn's disease is unknown, though research has identified likely sources of the ailment and eliminated some others.
The likeliest root culprit for Crohn's disease is an abnormal immune response in the body, likely triggered by a virus or bacteria. In attempting to fight off the trigger, your body also attacks the digestive system.
Heredity seems to play a role in the development of Crohn's disease, but is not the sole factor. You are more likely to develop Crohn's if you have relatives with the disease, but most people with it don't have a family history.
Eliminated causes of Crohn's include diet and stress. These contribute to the aggravation of Crohn's and worsening of symptoms in some patients, but do not directly cause the development of the disease.