Crohn's disease affects the digestive system. It usually affects the small intestine and large intestines, but can affect anywhere from the mouth to the bottom. People can also develop problems of other organs. It is caused by inflammation in the gut, and so it's known as one of the inflammatory bowel diseases.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/what-causes-crohns-disease