True lab mice are raised to be genetically pure for certain traits and have been strictly linebred in order to fix those traits or genes so that each individual within the specific strain is indistiguishable to the one next to it. They are virtual clones of each other. This is necessary so that each individual reacts the same way to the same experiments. They are often raised in ultraclean disease free environments and their immune systems are often unable to fight off simple diseases. I've known a few breeders who have picked up lab mice from biological supply houses that have lost the entire group when they mixed them in with their current population.