Had it this past week. Don't worry about it -- they definitely don't grill you. Just make sure you have a vague understanding about health care issues and how insurance companies can / do affect the way health care is distributed in America. And I mean this very broadly - know enough to have a solid opinion on the big-picture issues.
I read a book called 10 Excellent Reasons for National Health Care to prep and it was way overboard; I'm sure you'll be ready by reading wikipedia. And also, the MMI is super relaxed and a great way to get interviewed for 80 minutes. Good luck man!