It's a stroke of luck, or chance event, to find a terrific parking space on a rainy day, but no one feels lucky when their grandfather has a stroke, a sudden debilitating loss of oxygen to the brain.
Stroke is a high value word — many meanings, yet rarely misunderstood in context. You can swim a certain stroke (or just doggie paddle), you can stroke a cat's fur, you can figuratively stroke your boss's ego by flattering him, or you can have a stroke of brilliance and solve world hunger. Hopefully you won't suffer a stroke and need hospitalization. Its origin as a medical term goes back to the 1500s when it was shortened from "the Stroke of God's Hand."