The fancy rat (Rattus norvegicus domestica) is the domesticated form of Rattus norvegicus, the brown rat, and the most common species of rat kept as a pet. The name fancy rat derives from the idea of animal fancy (the promotion of domesticated animals) or the phrase "to fancy" (meaning to like or appreciate). Wild-caught specimens that become docile and are bred for many generations still fall under the fancy type. Fancy rats were originally targets for blood sport in 18th- and 19th-century Europe. Later bred as pets, they now come in a wide variety of coat colors and patterns, and are bred and raised by several rat enthusiast groups around the world. They are sold in pet stores and by breeders. Fancy rats are generally easy to care for provided plenty of research is done and are quite affordable, even compared to other small pets; this is one of their biggest draws. Additionally, they are quite independent, loyal and easily trained. They are considered more intelligent than other domesticated rodents. Healthy fancy rats typically live 2 to 3 years. Fancy rats are used widely in medical research, as their physiology is very similar to that of humans. When used in this field, they are referred to as laboratory rats (lab rats). Domesticated rats are physiologically and psychologically different from their wild relatives, and typically pose no more of a health risk than other common pets. For example, domesticated brown rats are not considered a disease threat, although exposure to wild rat populations could introduce pathogens like the bacteria Streptobacillus moniliformis into the home. Fancy rats have different health risks than their wild counterparts, and thus are unlikely to succumb to the same illnesses as wild rats. Rats are typically distinguished from mice by their size. Generally, when someone discovers a large muroid rodent, its common name includes the term rat, while if it is smaller, its name includes the term mouse. The common terms rat and mouse are not taxonomically specific.