Kansas is only one of many hypotheses, and is certainly not the consensus of the scientific community.
https://en.wikipedia.org/wiki/Spanish_flu#Hypotheses_about_the_source
Hypotheses about the source
United Kingdom
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.
A report published in 2016 in the Journal of the Chinese Medical Association found evidence that the 1918 virus had been circulating in the European armies for months and possibly years before the 1918 pandemic.
United States
There have been statements that the epidemic originated in the United States. Historian Alfred W. Crosby stated in 2003 that the flu originated in Kansas, and popular author John M. Barry described a January 1918 outbreak in Haskell County, Kansas, as the point of origin in his 2004 article.
A 2018 study of tissue slides and medical reports led by evolutionary biology professor Michael Worobey found evidence against the disease originating from Kansas as those cases were milder and had fewer deaths compared to the situation in New York City in the same time period. The study did find evidence through phylogenetic analyses that the virus likely had a North American origin, though it was not conclusive. In addition, the haemagglutinin glycoproteins of the virus suggest that it was around far prior to 1918 and other studies suggest that the reassortment of the H1N1 virus likely occurred in or around 1915.
China
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 due to lack of data during the Warlord Period of China, see Around the globe). Multiple studies have documented that 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 in 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.
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 and instead found evidence of its circulation in Europe before the pandemic. The 2016 study suggested that the low flu mortality rate (an estimated 1/1000) found among the Chinese and Southeast Asian workers in Europe meant that the deadly 1918 influenza pandemic could not have originated from those workers.
A 2018 study of tissue slides and medical reports led by evolutionary biology professor Michael Worobey found evidence against the disease being spread by Chinese workers, noting that workers entered Europe through other routes that did not result in detectable spread, making them unlikely to have been the original hosts.[26]
Other
Hannoun considered several alternative hypotheses of origin, such as Spain, Kansas, and Brest, as being possible, but not likely.[34] Political scientist Andrew Price-Smith published data from the Austrian archives suggesting the influenza began in Austria in early 1917.
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1918 Pandemic (H1N1 virus) (from the CDC)
https://www.cdc.gov/flu/pandemic-resources/1918-pandemic-h1n1.html
The 1918 influenza pandemic was the most severe pandemic in recent history. It was caused by an H1N1 virus with genes of avian origin. Although there is not universal consensus regarding where the virus originated, it spread worldwide during 1918-1919. In the United States, it was first identified in military personnel in spring 1918. It is estimated that about 500 million people or one-third of the world’s population became infected with this virus. The number of deaths was estimated to be at least 50 million worldwide with about 675,000 occurring in the United States.
The Deadliest Flu: The Complete Story of the Discovery and Reconstruction of the 1918 Pandemic Virus
https://www.cdc.gov/flu/pandemic-resources/reconstruction-1918-virus.html
Study revives bird origin for 1918 flu pandemic
https://www.nature.com/news/study-revives-bird-origin-for-1918-flu-pandemic-1.14723
The 1918 Spanish Flu Pandemic, the Origins of the H1N1-virus Strain, a Glance in History
https://www.researchgate.net/publication/314079733_The_1918_Spanish_Flu_Pandemic_the_Origins_of_the_H1N1-virus_Strain_a_Glance_in_History
The Origin and Virulence of the 1918 “Spanish” Influenza Virus
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2720273/
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