(An aneurysm is a swollen blood vessel, which can burst suddenly, though I'm sure [info]ithilwen can tell you much more.

Not really; there's not much more to tell. Some people are predisposed to them, and they usually can't be diagnosed before they rupture. If the person is lucky, it's a slow leak which causes nothing worse than an excruciating headache, and the aneurysm can be clamped off surgically. If they're not lucky...