Boy: Germs, oh, what're germs?
Doctor: The things that cause infection.
Boy: Infection is caused by germs? Oh, are germs poisonous?
Boy: I don't see any poisonous germs.
Doctor: Oh, much too small to be seen, but they're everywhere.
Boy: Oh, they are? Well you, you can't be too careful.
Prevention is better than cure. I'll say.
What does "I'll say" mean in this sentence?