Rhazes (rāˈzēz) [key] or Rasis rāˈsĭs,–zĭs, 860–932, Persian physician. He was chief physician at the Baghdad hospital. An observant clinician, he formulated the first known description of smallpox as distinguished from measles in a work known as Liber de pestilentia (tr. A Treatise on Smallpox and Measles, 1848). His works were widely circulated in Arabic, and Greek versions and were published in Latin in the 15th cent. They include a textbook of medicine called Almansor and an encyclopedia of medicine compiled posthumously from his papers and known as Liber continens.