In WWII, a farmer's son saved Churchill from drowning. What did this person later discover that saved millions of lives?trivia question
I think that what you are referring to here is the story that Sir Alexander Fleming saved Winston Churchill from drowning. This was supposed to have happened well before WWII, though. Fleming was supposed to have saved Churchill (indirectly, though) again in WWII. This was because Fleming was the person who discovered penicillin. Supposedly, Churchill was treated for a serious illness (with penicillin) and was thereby saved by Fleming again.
This story is not really widely believed by scholars any longer. Follow the churchill.org link to see what scholars say about the story.
So the answer is penicillin, but the story does not really seem to be true.