Blackcat:
The previous owner had some breeding mice that they allowed to all live together. Progressively the mice became more and more (as rodents do!) until there was close to 200 of them sharing their space. The males then ended up fighting over food, territory and females. At the last count, after they were separated and the females had had any babies it was closer to 400.
The fights resulted in things like missing tails, sometimes worse. Not all of the mice could be saved from the injuries they'd sustained as far as I'm aware.
Smudgey, to be honest I'm not entirely sure. I can PM you the email for the rescue lady I was in touch with if you want.