Rats are generally small animals. A typical rat, Rattus norvegicus or the Norway rat, is about 9 in (23 cm) from the nose to the base of the tail when fully grown and weighs about 2 lb (1.8 kg). One of the largest species, the southern giant slender-tailed cloud rat Phloeomys cumingi, has a head-body length of 19 in (48 cm) and a tail that ranges between 8-13 in (20-33 cm) long.
Rats have brown, gray, or black fur covering their body, except for their ears, tail, and feet (the familiar white lab rat is an albino form of R. norvegicus). Their hearing is excellent, and their eyes are suited for a nocturnal lifestyle. Rats typically have 16 teeth, the most prominent of which are their ever-growing incisors. The outer surface of the incisors is harder than the inner side, much like a chisel. The incisors grow throughout life from the base and are nerveless except for at the base. Rats must gnaw continually to keep the incisors down to a manageable length; if rats fail to gnaw, the teeth can grow rapidly and curl back into the roof of the mouth, or (with the lower incisors) up in front of the nose, making biting and eating difficult.
The teeth, combined with the rat's powerful jaw muscles, allow them to chew through almost anything; even concrete block and lead pipe have been found bearing toothmarks. The jaw muscles exert an extraordinary 24,000 lb (12 tons) per square inch (for comparison, a great white shark bites with a force of 20 tons per square inch). One of the masseter muscles responsible for this tremendous biting power in the rat passes through the orbit, or eye socket, a feature unique among the mammals.
A rat will bite a perceived enemy, particularly if cornered or if its nest is threatened. It also often bites out of curiosity, when exploring the edibility of unfamiliar things. Unfortunately, a sleeping child or unconscious derelict may be the subject of this investigation, with potentially serious consequences. Rats do carry a variety of zoonoses (animal-borne diseases) in their saliva, on their fur, and in their external parasites (such as fleas), that can and do infect humans. Best known are rat-bite fever and bubonic plague, transmitted to humans by rat saliva and rat fleas, respectively. When a rat walks though garbage in which salmonella bacteria are present, the microbes can latch onto the rat's fur. When the rat later investigates a pile of human food, the salmonella moves from the fur to the food, and whoever eats it may develop food poisoning.