John M. Barry in Smithsonian Magazine:
Haskell County, Kansas, lies in the southwest corner of the state, near Oklahoma and Colorado. In 1918 sod houses were still common, barely distinguishable from the treeless, dry prairie they were dug out of. It had been cattle country—a now bankrupt ranch once handled 30,000 head—but Haskell farmers also raised hogs, which is one possible clue to the origin of the crisis that would terrorize the world that year. Another clue is that the county sits on a major migratory flyway for 17 bird species, including sand hill cranes and mallards. Scientists today understand that bird influenza viruses, like human influenza viruses, can also infect hogs, and when a bird virus and a human virus infect the same pig cell, their different genes can be shuffled and exchanged like playing cards, resulting in a new, perhaps especially lethal, virus.
We cannot say for certain that that happened in 1918 in Haskell County, but we do know that an influenza outbreak struck in January, an outbreak so severe that, although influenza was not then a “reportable” disease, a local physician named Loring Miner—a large and imposing man, gruff, a player in local politics, who became a doctor before the acceptance of the germ theory of disease but whose intellectual curiosity had kept him abreast of scientific developments—went to the trouble of alerting the U.S. Public Health Service. The report itself no longer exists, but it stands as the first recorded notice anywhere in the world of unusual influenza activity that year.
More here.