What determines disease severity and location? A question that has been on my mind since I was first diagnosed five years ago.

As the saying goes: no two Crohn's patients are the same. But why?

There are people who were diagnosed with Crohn's ages ago and take no medications for it as the symptoms are not that bad. There are people who have their disease in the terminal ileum only. There are people who have their disease in the colon only. There are people who need a proctocolectomy because of the severity of the disease. There are people who get perianal disease and those who don't. There are people who forget about Crohn's after their first resection.

Why? Are there gene markers that determine how severe the disease is and what course it will take?