Both my mother and father lived through the 1918 World Wide Flu Epidemic. I do not know whether they had the actual flu or not - or were just naturally immune to it from their own genetic inheritance.
I have had the flu many times over my life... the Hong Kong flu in 1968 was very severe - I recall being extremely sick. I also had other flus in the 70’s, 80’s and 90’s - plus probably had Nile Fever in 2003.
But - I survived and I am probably better off immunologically than if I had not had the many cases of the flu.
Immunity is a complicated factor (to make an understatement) ... Those people today who have natural immunity to HIV/AIDS are mostly descendants of Europeans who survived the Bubonic Plague (Black Death). The survival gene ‘Delta 32’ is quite a story.
The Great Influenza (The Epic Story of the Deadliest Plague in History)
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I highly recommend this book as an excellent explanation of the reasons why the 1918 epidemic was so deadly.
Not only was it a new strain, but medical knowledge was inadequate, and political decisions interfered because of the call-up of troops for WWI.