What's the scariest thing to ever happen to you health-wise?

I had a kidney stone that got stuck in my bladder and continued to grow. The scary part, I thought it was just a kidney stone and every time I started passing blood I thought "ok, this is it, a couple of days and this will be over". Then it would stop and I'd feel better for a couple of weeks. Maybe. Then I'd lift something heavy or move wrong and start passing blood again.
This went on for about a year with the bad days getting more and more frequent before I decided I would never pass it and needed to see a dr. By that time it had grown to the size of an egg and I wasn't having any good days.