MD has explained it how we have explained it to DD.

We have explained that there are normal doctors you go to when you are a little bit sick and special doctors when the normal doctors can't help (all three of us have had surgery in the past 18 months do we've had a lot of medical appts, and all three if us in emergency!!). We've also lost a couple of family members in that time - both of them passed in hospital from drawn out illness, so we were able to explain about people being very very sixth and the doctors not being se to fix them. They can fix lots, but not everything.

We also used analogies she could understand by using toys. Some are broken that we can fix, and some are totally wrecked and can't be fixed.
It's hard for them to get a grasp if the differences when they are little, so simplifying as much as you can is helpful.