Several diseases, like Rat Coronavirus Infection (RCI), Sendai virus, and Murine Respiratory Mycoplasmosis (MRM, Mycoplasma pulmonis), are prevalent simply because their highly contagious natures work in tandem with the way rats are kept in laboratories, pet stores, and by breeders [REF].
There is some evidence that spayed female rats ("does") are less likely to develop mammary and pituitary tumors than non-spayed females [REF].