Acute viral infection of the respiratory tract. From Latin influentia, “to flow into”; in medieval times, intangible fluid given off by stars was believed to affect humans. The Italian influenza referred to any disease outbreak thought to be influenced by stars. In 1743, what Italians called an influenza di catarro (“epidemic of catarrh”) spread across Europe, and the disease came to be known in English as simply “influenza.”

Sources: Dorland’s illustrated medical dictionary. 30th ed. Philadelphia: Saunders; 2003 and Quinion M. World wide words. 1998 Jan 3 [cited 2005 Dec 5]. Available from http://www.worldwidewords.org/topicalwords/tw-inf1.htm