My point was the massive mobilization, and subsequent de-mobilization, of armies during World War I. Especially starting in 1917 across the Atlantic when the US entered the war in large numbers.

Sure, not comparable to the way are traveling today, but the numbers still were in the millions. And they were cramped into ships for weeks, as compared to aircraft. The Diamond Princess showed us that this a nice breeding ground for a virus.

EDIT:

Some numbers:

USA: 1.5 million men demobilized by February 1919, so it started right uring the second wave, in the US. Another roughly 2 million men in cramped demobilization camps in Europe. Later these US troops were moved from these camps to French towns and cities to ease tension among troop caused by crowded conditions. Also happening right during the second flu wave.

Germany: roughly one million men simply walked home after the armistice in Nov. 1918, leaving roughly 6 million men under arms. These were demobilized in the four month after the armistice. So you have 7 million men travelling across the German Empire. During the second wave of the flu.

Austria: No numbers found after a quick gogle search, except for 400,000 soldiers taken prisoner by the Italian during the desintegration of the Austr-Hungarian army. The demobilization in Austria collided with the desintagration of the Empire, so it was even more chaotic than the German one. Also taking place after the armestice in Nov. 1918 and during the second wave of the flu.

Great Britain: demobilized roughly 3 million men between the armistice in Nov. 1918 and late 1919. Mostly from the Western Front. During the second wave of the flu.

France: Demobilized 2.5 million men between Nov. 1918 and April 1919. Also from the western front, also during the second wave of the flu.

There we talk about young men, the demographic most ht by the flu. All war powers kept the flu under wraps during the war, that's why it's called the Spanish Flu, Spanish newpapers were the first to report it. All these men travelled, moreor less coordinated, across Europe and the US. Most of them coming from the Western Front, from cramped conditions with sometimes bad hygenic conditions. And they met up with the young female population upon return, for obvious reasons. All in the time frame of a couple of months.

The equivalent would be to move a large protion of our elderlies around the world now.

Source: https://encyclopedia.1914-1918-online.net/article/demobiliza...

EDIT 2: From the submitted article: "Historians now believe that the fatal severity of the Spanish flu’s “second wave” was caused by a mutated virus spread by wartime troop movements."

EDIT 3: It was much easier to find demobilization numbers thn mobiliztion numbers. The second wave hit before the Armistice in November. Demobilization is,if you want to call that, the return ticket of all these young people taking their trips to the western front. So they all moved there in the months before. And they travelled, from the trenches, to rear, on front vacation, to field hospitals, to hospitals elsewhere. They also travelled on the same ships that ran across the Atlantic, with much larger crews than we have today. The amount of traffic during the months before, and drung, the second wave was mind boggeling. Entente powers geared up for the huge, planned, 1919 spring offensive.

EDIT 4: Nice tid bit, not only for the second wave. "By the end of the summer the virus had reached the German Army. The virus created serious problems for the German military leadership as they found it impossible to replace their sick and dying soldiers. The infection had already reached Germany and over 400,000 civilians died of the disease in 1918." from here: https://spartacus-educational.com/FWWinfluenzia.htm.

I would have thought barbed wire, artillery, tranches, chemicla warfare and machine guns would have formed a nice barrier. Social contact, trenches have a lot of that, seemto be a much bigger factor it seems. Proves the point of avoiding gatherings, self-quarantine and sch things, IMHO.