True rats are rodents that belong to the Rattus genus of the family Muridae, though the term is often used in a more general way. Native to Asia, these small mammals have spread worldwide by traveling with humans. The most familiar varieties are two types of house rat, Rattus norvegicus, which is commonly called the brown or Norway rat, and Rattus rattus, better known as the black or roof rat. Of these species, the brown rat is the larger, though it is the black rat that has perhaps caused the most damage. The black rat is the species that was common during the Middle Ages in Europe and which played a central role in the spread of the devastating bubonic plague. Contrariwise, the brown rat has been extremely useful to humans in the sense that they served as the basis for the albino strain of rats used prevalently in research laboratories.