Rats are primarily nocturnal although a major exception is the brown rat, which is active during both day and night. "Rat" is a general term applied to many members of different rodent families. In scientific terms, rats include 56 species of medium-sized, thin-tailed rodents.
Rats are prolific animals, originating in Asia and spreading nearly everywhere humans live. They are often responsible for killing poultry, eating stored food and spreading disease. Many indigenous species of other animals are extinct due to the introduction of rats. While rats have a negative reputation, their use in the medical field has led to many scientific breakthroughs.