Cotton rat: A rodent capable of carrying the types of hantavirus that cause HPS (hantavirus pulmonary syndrome) in the US. The cotton rate is found in the southeastern US and Central and South America. Its head and body measure about 5 to 7 inches with another 3 to 4 inches for the tail. The hair is longer and coarser, of a grayish brown color, even grayish black. The cotton rat prefers overgrown areas with shrubs and tall grasses. Scientific name: Sigmodon hispidus.