Answer
Smallpox and measles are two infectious diseases that were clinically distinguished by the Persian physician Abu Bakr al-Razi in the 9th century. In his book Kitab al-Judari wa al-Hasbah, al-Razi described the differences in the symptoms, prognosis, and treatment of the two diseases, pioneering early efforts in differential diagnosis and contributing significantly to the development of medical knowledge during the Islamic Golden Age.