In the 7th century ad a vast portion of the Eastern world was overrun by Arab conquerors. In Persia (now Iran), the Arabs learned of Greek medicine at the schools of the Nestorian Christians (see Nestorianism), a sect in exile from the Byzantine Empire. These schools had preserved many texts lost in the destruction of the Alexandria Library. Translations from Greek were instrumental in the development of an Arabic system of medicine throughout the Arab-speaking world. Followers of the system, known as Arabists, did much to elevate professional standards by insisting on examinations for physicians before licensure. They introduced numerous therapeutic chemical substances and excelled in the fields of ophthalmology and public hygiene.
Important among Arabist physicians was al-Razi, who was the first to identify smallpox and measles and to suggest blood as the cause of infectious diseases. Avenzoar was the first to describe the parasite causing the skin disease scabies and was among the earliest to question the authority of Galen. Maimonides wrote extensively on diet, hygiene, and toxicology, the study of chemicals and their effect on the body. Al-Quarashi, also known as Ibn al-Nafis, wrote commentaries on the writings of Hippocrates and treatises on diet and eye diseases. He was the first to determine the pathway of blood, from the right to the left ventricle via the lungs.