In 1917 and 1918, the electron microscope was not even a dream. The best antibiotic they had was sulfa for exterior wounds. Penicillin was to come later. Physicians believed in a body having a "Nexus Of Infection" a disease center.
Old people die easier because they are frail to begin with.
Young people can die because they hadn't yet been exposed to a pathogen similar to the virus ravaging the populace. A friend in 2010 laughed when I told him how bad the influenza outbreak was. A week later he was in the hospital on a ventilator. This was the first time in his 60, years of age he ever contracted the flu.
Viruses and Pneumococcal bacteria can tag-team in seeming endless varieties. A particular virus like H5N1 can pair with a particular virulent Pneumococcal pathogen and produce another pandemic similar to the one a hundred years ago. The so-called "Bird Flu" is one that can adapt a piggyback bacterium that would be particularly dangerous.
Rhino and Influenza viruses mutate. It was a mutation that abruptly ended the "Spanish Flu" pandemic.
Regardless, today's immunologists would scoff at the idea that a virus alone could cause a deadly pandemic. The virus would have to pair with Pneumococcal bacteria to cause widespread deaths. Sepsis can cause meningitis, renal and hepatic failure. Septicemia can kill a person WITHIN HOURS.
Four of us, three males in three households have had our pneumonia inoculations. I twisted Eduardo's and Orlando's arms to get theirs, last September. Together, in two large homes they have FOURTEEN family members. Ranging in age from 2-years to seventy. Guess what EVERY LAST ONE OF THEM except for Eduardo and Orlando have very sore throats as of 7:00PM this evening, high fevers and bronchial congestion. Eduardo is symptom free. Orlando has the same degree of sickness as I. Celso the fourth also has light symptoms. He has no cough, I have a so-called productive cough that is minor.
Each to their own. I still remember "Pete" staring at me while being ventilated. I was allowed to stay for a couple of minutes. That year the Mexican vaccine came from Switzerland and was different than the USA vaccine.
Three days into this flu (a cold does not produce a fever*) I felt well enough to go to Home Depot and purchase wire to finish a minor project. I had alcohol gel for my hands and used it liberally.
100.3F that lasted three hours last night.