Older people who had had the actual flu or a vaccine still showed anti-bodies to various strains of the flu...
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My mother had the 1918 flu when she was 3 years old. I have never had a flu infection, and maintain that I got a powerful immunity from her milk. It seems like I have a master immunity to all the subdivisions/mutations that have occurred since 1918.
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.