https://www.kshs.org/kansapedia/flu-epidemic-of-1918/17805
"In 1918 the United States was involved in World War I, but was also dealing with the outbreak of a deadly influenza epidemic. The first cases of the outbreak were recorded in Haskell County, Kansas, and Fort Riley, Kansas, where young men were being hospitalized for severe flu-like symptoms. A local doctor sent a report to the Public Health Service, but no one was sent to investigate the situation. On March 4, 1918, an outbreak appeared at Fort Riley, with as many as 500 soldiers hospitalized within a week. Within a month, however, the number of patients dwindled and it seemed that the flu had passed its course. Many of these soldiers were sent to Europe to help fight in World War I. While in Europe the disease mutated and became deadly. By May many reports of soldiers falling ill were reaching the U.S. It did not take long for the disease to spread from the soldiers to the civilian population of Europe, and then around the world. Few areas remained unaffected, and there were recorded outbreaks in Asia, Africa, Europe, North and South America, as well as the Arctic and remote Pacific Islands. ...
A third and final wave of the epidemic hit in the spring of 1919, and many reported that it was so severe that people could wake up healthy and be dead by nightfall. By the end of spring the number of patients had dropped enough that officials lifted bans from their cities and states and people could resume school and church. Since the disease occurred at the same time as World War I, the epidemic was overshadowed. Although the epidemic only lasted a year, it left a large mark, both in America and worldwide."
That’s one common hypothesis put forward, along with European, African and as I said Chinese origin. However, there have been recent developments since the publishing of “The Great Influenza” that have IMO lent a little more credence to it being from China, such as DNA analysis showing similarities to viruses still wandering around mostly in that part of the world.
The final truth of the matter is that nobody knows for certain where the bloody thing came from for certain and we will probably never know. There is, however, new evidence coming to light all the time - for example, research has uncovered that there *may* have been an earlier form of the virus afflicting British soldiers in England at least as early as 1917. https://www.statnews.com/2018/12/05/1918-spanish-flu-unraveling-mystery/
Here is research regarding the Chinese theory: https://www.nationalgeographic.com/news/2014/1/140123-spanish-flu-1918-china-origins-pandemic-science-health/
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A decade after the war, Kansas was identified as another possible breeding ground, due to reports of an influenza outbreak there that spread to a nearby Army camp in March 1918, killing 48 doughboys.
But in his study, Humphries reports that an outbreak of respiratory infections, which at the time were dubbed an endemic “winter sickness” by local health officials, were causing dozens of deaths a day in villages along China’s Great Wall. The illness spread 300 miles (500 kilometers) in six weeks’ time in late 1917.
At first thought to be pneumonic plague, the disease killed at a far lower rate than is typical for that disease.
Humphries discovered that a British legation official in China wrote that the disease was actually influenza, in a 1918 report. Humphries made the findings in searches of Canadian and British historical archives that contain the wartime records of the Chinese Labor Corps and the British legation in Beijing.
Sealed Railcars
At the time of the outbreak, British and French officials were forming the Chinese Labor Corps, which eventually shipped some 94,000 laborers from northern China to southern England and France during the war.
“The idea was to free up soldiers to head to the front at a time when they were desperate for manpower,” Humphries says.
Shipping the laborers around Africa was too time-consuming and tied up too much shipping, so British officials turned to shipping the laborers to Vancouver on the Canadian West Coast and sending them by train to Halifax on the East Coast, from which they could be sent to Europe.
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This predates the Kansas incident.