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A letter discussing the flu epidemic, November 1918. Courtesy of the University of Utah J. Willard Marriott Library via the Mountain West Digital Library.

The 1918-1919 influenza pandemic was unique because it came in three waves, whereas previous outbreaks were characterized by a single wave with limited geographic dispersion. This pandemic spread quickly and globally, claiming an estimated 50 million lives, among them some 500,000 Americans. In the US, the first wave appeared in the spring of 1918, the second in the fall of 1918, and the third in the early winter of 1919. Of the three waves, the second was most fatal, which led to a theory that survivors of the second wave had developed protective immunity.

Like the mystery surrounding its biological origins, there are many theories about where the influenza started and how it spread. The dominant theory at the time claimed it began in Spain in April 1918, as evidenced by the large number of afflicted people documented there early on. This led to the misnomer "Spanish Flu." Another theory hypothesized that the flu originated at Fort Riley, Kansas, and spread via soldiers wherever they were deployed. Yet another theory suggested that soldiers brought it home from China where it quickly spread to US civilians. Despite numerous speculations, scientists have yet to determine the precise geographic origin of the disease.