Why "influenza" is named after the stars
Long before modern virology, medieval Italians believed that sudden outbreaks of disease were caused by cosmic forces. The word influenza comes from the Latin influentia, meaning "influence." People believed that a malevolent "influence" flowing from the stars and planets was responsible for causing the sudden epidemics of high fever and coughing.
The Celestial Origin of a Word
The English word influenza comes directly from Italian, which borrowed the term from the medieval Latin influentia. In its earliest medieval usage, the word had little to do with coughing, fever, or respiratory distress. Instead, influentia referred to an ethereal fluid or invisible force thought to flow down from the heavens, emanating from the stars and planets into the terrestrial realm to shape human events, temperaments, and physical constitutions.
When sudden, sweeping illnesses struck entire towns at once without an obvious physical cause, medieval observers sought explanations in the sky. Because an epidemic arrived swiftly, affected thousands across a vast geographic area, and then disappeared just as mysteriously, people attributed the catastrophe to an occult influence—an unfavorable alignment of heavenly bodies that cast a malevolent vapor or imbalance upon the population below.
Medieval Medicine and the Logic of Cosmic Influence
In the centuries before the development of germ theory, medicine was closely intertwined with natural philosophy, humoral theory, and astrology. Classical and medieval physicians believed that human health depended on maintaining a delicate balance among four bodily humors: blood, phlegm, yellow bile, and black bile. These humors were thought to be continuously affected by environmental factors, including seasonal weather patterns, regional winds, and the movements of the planets.
Astrological medicine posited that unusual planetary conjunctions or celestial anomalies, such as comets, could corrupt the air or directly alter the humoral balance of human bodies. An outbreak of acute fever and respiratory illness that swept across entire regions was therefore labeled an influenza delle stelle—an "influence of the stars." The phrasing reflected an earnest attempt to apply the prevailing scientific framework of the era to explain why large swathes of humanity fell ill simultaneously.