Does anyone know the cause of high mortality in the 20-40 age group?
Also: how much faster does a pandemic move now compared to 100 years ago (Airplanes, population density)?
Also: how much faster does a pandemic move now compared to 100 years ago (Airplanes, population density)?
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.
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.
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
The latest edition of Ronan Farrow's Catch and Kill podcast describes how NBC agreed to censor his story on Harvey Weinstein. [https://podtail.com/podcast/the-catch-and-kill-podcast/episo...]
>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.
https://www.cdc.gov/flu/pandemic-resources/reconstruction-19...
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.
https://pubmed.ncbi.nlm.nih.gov/30349811-back-to-the-future-...
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.