As others have said, the current understanding is that it was cytokine storms. Your immune system responds to an infection by creating white blood cells which in turn release cytokines to cause inflammation.
That's normally a helpful response and part of how your immune system fights infections. But it also involves raising your body temperature, breaking down cells, and changing blood pressure. When those processes go too far, they can end up hurting or killing the organism by taking out too many healthy cells or interfering with circulation.
The Spanish flu triggered a very strong immunological response. So you had a mortality spike around 20-40 years old because those were the people with such healthy immune systems that their inflammation over-reacted and killed them.
By analogy, it's sort of like your house catching on fire. Hundreds and hundreds of fire fighters pile in, so many that they building actually collapses from their weight.
According to "The Great Influenza" by John Barry young people died due to cytokine storms. Essentially young people have a vigorous immune system that gave an aggressive attack. 1918 flu was unique in that the deaths was not just the very young and the very old (a U shaped distribution) but rather a W shaped distribution with the middle point being 20-40 age group
Also in the book - the "spanish" flu was called that since Spain was the only place with a free press at the time (e.g., US press was not free at the time) + massing of young men in US in camps - like Ft Devens. Flu traveled from overcrowded camp to camp and then jumped to local populations along rail lines (and then overseas as troops were shipped out)
He describes it well, though would make a great visual as it moves from Boston to Chicago to NYC back into Providence/Brockton (which are just south of Boston). Transmission along rail (and shipping) lines.
Yes, this was a big reason why it was initially named the Spanish Flu. At the time that it began (with the war still on) Spain was one of the few countries that did not have very stringent press censorship in place.
This is discussed in several sources, but this is the one I had most readily available:
"In a month or two everyone outside of Spain was calling it "Spanish influenza," not because it originated there, but probably because Spain, still a nonbelligerent, had no wartime censorship to keep its health problems secret from the world. An estimated eight million Spaniards caught flu in May and June. The Spanish claimed that it had come from the battlefields of France, blown over by the strong winter winds, and that it would have been even worse but for the snowy Pyrenees."
America's Forgotten Pandemic: The Influenza of 1918 by Alfred W. Crosby
I don't have a ready source, but the press in the US would usually comply with a request to censor news that some in government worried would lead to panic, blame, or accountability. Continued well into the 60's and 70's, and still present today.
But IIRC there were dramatically more strict restrictions during WWI. Certainly in all the european countries busy fighting. (Famously, Bertrand Russell got himself locked up for six months for giving a lecture about whether the US should enter the war.)
>Laboratory studies on the reconstructed 1918 virus began in August 2005. A report of this work, “Characterization of the Reconstructed 1918 Spanish Influenza Pandemic Virus”, was published in the October 7, 2005, issue of Science.14 To evaluate the 1918 virus’ pathogenicity (i.e., the ability of the virus to cause disease and harm a host), animal studies involving mice were conducted. The mice were infected with the 1918 virus, and measures of morbidity (i.e., weight loss, virus replication, and 50% lethal dose titers) were collected and documented. For comparison, other mice were infected with different influenza viruses that were designed via reverse genetics to have varying combinations of genes from the 1918 virus and contemporary human seasonal influenza A(H1N1) viruses. These viruses are called “recombinant viruses.”
>The fully reconstructed 1918 virus was striking in terms of its ability to quickly replicate, i.e., make copies of itself and spread infection in the lungs of infected mice. For example, four days after infection, the amount of 1918 virus found in the lung tissue of infected mice was 39,000 times higher than that produced by one of the comparison recombinant flu viruses.
The referenced page goes on to detail how various parts of the 1918 virus contributed to virulence. I've seen articles that said that the reason 20-40 year old patient had more severe disease is that their immune systems were stronger, and that the lung damage is actually caused by the immune system combating the virus.
The 1918 virus started out earlier during WW I. During that time, large groups of men were being transported internationally by troop ships. While trains and ships are slower than airplanes a journey from a camp in the US to one in France probably took under two weeks.
Aside from the other points people have made, the main hypothesis is that the 1918 flu was similar to previous pandemics which the older were exposed to when they were young, so they already had some resistance. The 20-40 year olds were unlucky in that this was their first exposure.
It's worth mentioning that the article doesn't specify if that age group had a higher mortality % per infected, or just a higher mortality total. I'm assuming they meant the first, anyone know for sure?
Also: how much faster does a pandemic move now compared to 100 years ago (Airplanes, population density)?