It started on ships crammed full with soldiers being repatriated from the trenches. A bit like Easyjet flights to Spain of the day. But it was most dangerous to those under 25 I understand. Hence a lot of repatriated soldiers got sick and spread it when they got back in the US or all parts of the then British Empire.
There was a documentary out there where hundreds on a ship that took a week or so to get from one point to another died mid sail with no way to tackle it in cramped conditions that just let it rip,
Rather like where we are headed in the next few weeks by inviting such catch covid parties to celebrate freedom day.
That same documentary also explained how many lives were saved in Manchester by the chief medic at the time imposing lockdowns that were not then a known thing but that clearly stalled the spread. I had no idea the city was the origin of this tactic, But they had little medical ways out then of course. So anything that helped was like the vaccine of the day.