Perhaps the 1918 flu gave some immunity to those who survived and passed that along to the family genes.
Also, Mexico isn't the cleanest areas and it appears that people who are already weakened from living in a polluted area who be prime suspects of being killed from a virus.
I've wondered about that too. Both my parents were born at that time, one in 1917 and one in 1918.
Some populations obstinately refuse hygiene as 'Western' influence. Mexico and India spring to mind as special biohazard areas.
These countries have educated medical professionals, and the authorities understand germ theory, yet they still lack basic sanitation and safe drinking water and no effort forthcoming in addressing these problems.
“Perhaps the 1918 flu gave some immunity to those who survived and passed that along to the family genes.”
What you suggest is known as acquisition of acquired traits. Like if you keep cutting the wings off generation-after-generation of fruit flies, and expecting there to be a genetic change so that eventually fruit flies are born without wings. It doesn’t work.