Mice, rats, and other small animals are supplied to pet stores by mass breeders and shipped in small, cramped containers that are breeding grounds for parasites and viral and bacterial infections. They often reach the pet store ill, malnourished, and/or pregnant.
So, ♥ rats and mice? Always adopt, and never buy them from pet stores that sell them! Also, be sure to write to PetSmart and PETCO today and demand that they stop selling all animals immediately!