Imagine you’re back in pre-school.
You’re sitting on the rug, listening to the teacher read a storybook. Suddenly, the nurse calls into the classroom. “Mrs. Jones? Can you send Bobby to the health office right away?”
You’re not sick, and you don’t take any medicines at school like your friend Michael does. Why do you have to go to the nurse?
When you arrive, the nurse tells you that someone else in your class has come down with a sickness called RSV. She can’t say who, but she knows you sit next to him at lunch. So he might have given you RSV, even if you don’t feel yucky yet. 
…