The human body is truly amazing. It allows us to sense the world around us, to do work and have fun, and to move from place to place. In fact, the human body does its work so well that most people don't think about it very much - until they get sick.
The germs (致病菌) that make people sick are everywhere. You can't see them, but they're there. They're sitting on your desk.
They're hiding on your computer's keyboard. They're even in the air that you are breathing.