For many years, medical historians and epidemiologists hypothesized that the outbreak could have started at a British army base in Étaples, France, or at Fort Riley in Kansas, where the first American cases of this new strain of flu were recorded in March of 1918.
More recently, experts have proposed a third hypothesis: The Spanish flu originated somewhere in northern China in late 1917 and swiftly moved to western Europe with the 140,000 Chinese laborers the French and British governments recruited to perform manual labor to free up troops for wartime duty.
https://www.history.com/news/why-was-the-1918-influenza-pandemic-called-the-spanish-flu