context: Although there are many theories about what causes Crohn's disease, none of them have been proven. There is a benefit, though, in understanding the possible causes of Crohn's disease and how they interact with one another. Doing so can help one better understand the symptoms, diagnosis, and treatment of Crohn's disease. Scientists believe that Crohn's disease is caused by a combination of these factors: Immune system problems Genetics Environmental factors How might each of these factors contribute to Crohn's disease? Read on to learn more. Scientists have linked immune system problems to inflammatory bowel disease ( IBD), including Crohn's. Usually, cells of the immune system defend the body from harmful microbes -- bacteria, viruses, fungi, and other foreign substances -- that have entered it. The body doesn't usually respond to all microbes, however. Many microbes are helpful, especially for digestion. And so the immune system leaves them alone. If there is an invader that needs to be eliminated, your body's defense reaction begins. This immune system response causes inflammation. Immune system cells, chemicals, and fluids flood to the site to overcome the offending substance. After the substance has been disabled or removed, the immune response ends. Inflammation subsides. For some reason, though, people with Crohn's disease have an immune system that reacts inappropriately. The immune system may be defending the body against helpful microbes by mistake. Or, for some other reason, the inflammatory response simply will not stop. Either way, over time, this chronic inflammation in the digestive system can result in ulcers and other injuries to the intestines. Brothers, sisters, children, and parents of persons with IBD, including Crohn's disease, are slightly more likely to develop the disease themselves. About 10% to 20% of people with Crohn's disease have at least one other family member who also has the disease. The condition is more common in certain ethnic groups, such as Jews, and is more prevalent in Caucasians. Scientists have identified a gene associated with Crohn's disease. This gene helps the body decide how to react to certain microbes. If the gene has changed or mutated in some way, your body's reaction to microbes may also be different from the normal reaction. Over time, IBD or Crohn's disease may develop. People with Crohn's disease have this mutated gene twice as often as people who do not have the disease. Environmental factors may help trigger Crohn's disease. Also, it should be noted that because a potential trigger is linked or associated to a condition does not mean that it causes it. Associated environmental factors may include any of the following: Substances from something you've eaten Microbes such as bacteria or viruses Cigarette smoke Other substances that are yet unknown Environmental factors may contribute to Crohn's disease in one of these two ways: They may trigger an immune system response. Once started, the response cannot stop. They may directly damage the lining of the intestines. This may cause Crohn's disease to begin or to speed up. The factors involved in causing Crohn's disease are complex. Scientists continue to seek more information about the causes -- in hopes of finding better ways to diagnose, treat, and perhaps even cure this frustrating and painful disease. In the meantime, understanding current theories about the causes of Crohn's disease can help you work with your doctor to explore how various treatments might work to control this condition.