There were three waves, spaced roughly five months apart.
The first wave was relatively mild. A few died but most recovered. The second wave was a mutated strain. From what we have been able to piece together, a mutation originated in Europe that had the unfortunate and unusual trait of being more deadly than its predecessor.
It was very deadly. So deadly, in fact, that it should have wiped itself out but troop movements at the end of World War One fed new populations to the virus all over the world. You could show first symptoms in the morning and be dead by midnight.
By August of 1918, it was clear that a wave of death was coming. October of 1918 was the worst month. Americans practiced social distancing on a grand scale. The streets of every major city were deserted. People stayed home.
The thing that made this so terrible was the age range for death. We were used to flu killing the very young and very old. Pandemic flu spiked right in the middle: young healthy 28 year olds were the most at risk.
Whole families were decimated. Sometimes one child would survive but no neighbors would visit the house to collect the survivor out of fear, leaving him or her to starve. Entire villages in Alaska perished.
And then it faded. Quickly. It killed so many, so fast, that it burned out. Or so we thought.
It came back in the spring of 1919. No troop movements accelerated it this time. But it was just as deadly to those who caught it. Survivors didn’t enjoy the experience very much. This was before the days of modern medicine. Very high fever, body aches, nausea, delirium. The experience sucked badly.
President Woodrow Wilson had it in the spring of 1919 while attending the Paris conference that, in the end, decided to exact harsh terms on Germany.
Many historians consider that conference to be the match that ultimately started World War two. Wilson opposed many of the conditions put on Germany but was too weak to get his concerns heard.
Woodrow Wilson’s Case of the Flu, and How Pandemics Change History
Then the pandemic burned out. It was over. Too many got sick too quickly. It burned through its fuel and died out.
Unfortunately our coronavirus pandemic will not burn out in the same way. At least that appears unlikely.