"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
