I did a research paper about Woman's suffrage a couple of semesters ago and used the pandemic as one of the reasons the movement was successful at that time. During my research I would find horrible stories about how fast the flu killed, one example was of a college student who called a hospital to tell them that she just found her roommates really sick and she needed help. The hospital asked her how she felt and she said she felt fine. She was told to get out of the room and they would send an ambulance. When the ambulance arrived less than 20 minutes later they found everyone in the room dead including the caller who still had the phone in her hand.
It is interesting, in that the younger and healthier you were, the faster you died.
The mechanism was an overcharged autoimmune response to the virus, which filled the lungs with fluid.
So if you were old, or otherwise had a compromised immune system, you sneezed twice and felt no other effects.
This virus actually ended WWI, BTW. Nobody could field healthy troops anymore. Not even the US - a shp would leave for Europe, and half of the soldiers would die of the flu before it got there.