My sister's teenage friend (she was 17 at the time, and plenty old enough to know better).
Dozy girl: "No, don't go out, you'll catch a cold if you get wet in the rain, you need to put a coat on!"
Me: "Umm... you do know that a cold is caused by something called a virus, right? And that you have to catch this virus off other people?"
Dozy girl looks perplexed, then turns to sister: "He's making it up, isn't he?"