How did Peter and I raise a germaphobe?

Sammy amazes me with her fear of disease. Any disease; she doesn't discriminate infectious from otherwise. Once, we heard a radio commercial about a study for people with diabetes. Without thinking, I explained diabetes to her in kids terms. She looked as if I had just told her some terrible path her life would take. Later, a different radio commercial mentioned the word diabetes, and she covered her ears and whimpered.
So I picked up some books at the library for the summer reading program. Sammy likes chapter books, particularly silly ones or mysteries. Junie B has been a favorite character, so I was surprised when she told me she didn't want to read the book I brought home. "I looked at it, and it's about germs. I don't like germs." Seriously, I think she would find a story about the flu more frightening than one about monsters.
She's been at camp this week. It's "Twilight Camp". Like day camp, but in the evening. They had an opportunity to go creek stomping. Sammy saw some poison ivy and used her shoe to move it away from her. When I picked her up, she was concerned about this, so I told her we would wash her legs off really well when we got home. (It turned out when I did try to wash her leg, only the bottom of her shoe touched the plant). Anyway, in the car, I asked her about the rest of her evening. They had s'mores. Yum. She couldn't remember whether or not she finished hers. I asked if she wasn't hungry or not feeling well. She was thinking about the poison ivy too much to eat her s'more. I swear this child will have ulcers before middle school. I mean, I know I worry about everything, but really? I tried to reassure her that even though it was called "poison" ivy, it really only made you itchy, not sick.
On the one hand, I'm glad I don't have to battle her over hand washing the way I do Charlotte. But the child stresses too much, and I want her to enjoy her childhood! Would it help if she understood the different kinds of diseases? Or would she become obsessive about sterileness? What about chemaphobia? I really don't want to go there! She wanted to eat some fresh picked blueberries today, and I told her to eat the ones I had washed. Do I explain pesticides too? Where does she get all this worry from? ;)

As I said, no such issues with Charlotte. Instead, we have the problem with her saying she has washed her hands when she hasn't, or only wetting the tips of her fingers!