"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
For the mystery of lawlessness is already at work. And the name of the lawless one will be revealed. Not everyone who calls to me saying, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the ones who do the will of my Father. Many will say, ‘Lord, did we not prophesy in your name? Did we not drive out demons in your name? Did we not do mighty deeds in your name?’ Then I will declare to them solemnly, ‘I never knew you. Depart from me, you servants of lawlessness.'
Matthew 7:21-23