If you have lots of rats running around there are two solutions
1. You can place corridor items (drinks machines, plants, ect) to get rid of the mouse holes.
2. You can do it the hard way and find out a route where the mice run. Place you cursor on the path of the mice, and when a mouse comes past you can click constintly and you will hit the mouse running past.
If you hit enough mice in your hospital, when you win the level you will be assigned to a secret mission where there are millions of mice running around.
note: the secret level has only happened to me once but im sure that it works
good luck