"Besides replicating very quickly, the 1918 strain seems to trigger a particularly intense response from the immune system, including a ‘cytokine storm’ – the rapid release of immune cells and inflammatory molecules.
Although a robust immune response should help us fight infection, an over-reaction of this kind can overload the body, leading to severe inflammation and a build-up of fluid in the lungs that could increase the chance of secondary infections.
The cytokine storm might help to explain why young, healthy adults – who normally find it easier to shake off flu – were the worst affected, since in this case their stronger immune systems created an even more severe cytokine storm."
BBC, Why the Flu of 1918 Was So Deadly
"You have accepted things you would not have accepted five years ago, a year ago, things that your father, even in Germany, could not have imagined. Suddenly it all comes to be, all at once. You see what you are, what you have done, or more accurately what you haven’t done. For that was all that was required of most of us: that we do nothing. You remember everything now, and your heart breaks. Too late. You are compromised beyond repair."
Milton Mayer, They Thought They Were Free