Are fruit flies bad for our health?

No, the only reason you would want to trap them is if they irritate you. Actually, there is nothing harmful about them.

They are good flies!

How come the flies don’t find their way out ?!!

If your hole is too big they will. Once the fruit fly crawls in, it can't seem to get out. You would think they would just fly back out through the holes, but they won't!.