https://en.wikipedia.org/wiki/Spanish_flu#Hypotheses_about_the_source
Historical and epidemiological data are inadequate to identify with certainty the pandemic's geographic origin. Different hypotheses have been made about it, with the three main ones being Kansas in the United States, a British army base in France, and northern China. Historian Alfred W. Crosby stated that the flu originated in Kansas, and popular author John Barry described Haskell County, Kansas, as the point of origin. It has also been stated that, by late 1917, there had already been a first wave of the epidemic in at least 14 US military camps.
The major UK troop staging and hospital camp in Étaples in France has been theorized by researchers as being at the center of the Spanish flu. The research was published in 1999 by a British team, led by virologist John Oxford. In late 1917, military pathologists reported the onset of a new disease with high mortality that they later recognized as the flu. The overcrowded camp and hospital was an ideal site for the spreading of a respiratory virus. The hospital treated thousands of victims of chemical attacks, and other casualties of war, and 100,000 soldiers passed through the camp every day. It also was home to a piggery, and poultry was regularly brought in for food supplies from surrounding villages. Oxford and his team postulated that a significant precursor virus, harbored in birds, mutated and then migrated to pigs kept near the front.
One of the few regions of the world seemingly less affected by the 1918 flu pandemic was China, where there may have been a comparatively mild flu season in 1918 (although this is disputed, see
Around the globe). There were relatively few deaths from the flu in China compared to other regions of the world. This has led to speculation that the 1918 flu pandemic originated from China. The relatively mild flu season and lower rates of flu mortality in China in 1918 may be explained due to the fact that the Chinese population had already possessed acquired immunity to the flu virus.
In 1993, Claude Hannoun, the leading expert on the 1918 flu for the Pasteur Institute, asserted the former virus was likely to have come from China. It then mutated in the United States near Boston and from there spread to Brest, France, Europe's battlefields, Europe, and the world with Allied soldiers and sailors as the main disseminators. He considered several other hypotheses of origin, such as Spain, Kansas and Brest, as being possible, but not likely. Political scientist Andrew Price-Smith published data from the Austrian archives suggesting the influenza had earlier origins, beginning in Austria in early 1917.
In 2014, historian Mark Humphries argued that the mobilization of 96,000 Chinese laborers to work behind the British and French lines might have been the source of the pandemic. Humphries, of the Memorial University of Newfoundland in St. John's, based his conclusions on newly unearthed records. He found archival evidence that a respiratory illness that struck northern China in November 1917 was identified a year later by Chinese health officials as identical to the Spanish flu. A report published in 2016 in the Journal of the Chinese Medical Association found no evidence that the 1918 virus was imported to Europe via Chinese and Southeast Asian soldiers and workers. It found evidence that the virus had been circulating in the European armies for months and possibly years before the 1918 pandemic.
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