Reliable records of influenza, dating back to the 1700s, suggest a pattern of one major pandemic every century. Among the pandemics for which there is solid documentary evidence, the outbreak of 1918-1920 is by far the greatest. The so-called Spanish Lady caused somewhere between 25 and 100 million deaths worldwide. It is distinctive both for its high mortality rate, in comparison to other flu pandemics, and for its unusual demographic effect: whereas the flu typically targets the very young and old, the 1918-1920 epidemic struck adults in the prime of life. Without a cure for the disease, public health authorities today are in a position to learn from the successes and failures of the early-twentieth-century response.