There is likely no public place in the world that is truly "clean". Have you ever used a public phone?

The real question is why don't people get sick a lot more than they do. The answer is our immune system. If you are basically a healthy individual who does not isolate yourself from the outside world, you have probably been exposed to most of the germs that can cause problems and are self-immunized against them. Why do tourists get sick in many countries while the natives don't? Same reason.

You are actually better off exposing yourself to the non-sterilized world in many cases than being overly protective. That doesn't mean you shouldn't try to be as clean as possible, but medical science tells us it is not necessary to be excessively fanatic about this issue in most cases.