Most companies I know of make your insurance effective through the end of the month that you term in, but they don't always have to and I've seen ones where they don't do it that way, so you need to ask. I agree with everyone else that it won't be a pre-existing condition unless you let your coverage lapse. Take COBRA until you find other coverage. By all means, do not let your coverage lapse if you can at all avoid it, as then you will be paying dearly for an crohn's related issues (or issues the insurance company may
say are crohn's related) in the future.