Galen of Pergamon (129-216): Greek-Roman physician.
Galen of Pergamon started his career as physician of gladiators, but he became court physician of the Roman emperors Marcus Aurelius, Commodus, and Septimius Severus. He published many books, which he had to rewrite after his library was destroyed by fire in 191.
These book became extremely influential in the Middle Ages, when they were well-read in both the Byzantine empire and the Arab world. Eminent scientists like Ibn Sina, Ibn Rushd and Vesalius based their researches on the foundations laid by Galen. Being both a doctor and a writer, he was rather indiscreet, but on the other hand: we now have the entire medical status of two Roman emperors.