1918 flu, corrected

People have wondered why there have been no further outbreaks of the 1918 flu.
Of course, one reason would be that all flu viruses are always changing.
But I wonder if another reason could be that so many people who had a genetic susceptibility to this strain of flu died from it that the population is less susceptible to it now.
It appears that the reason cystic fibosis is so prevalent among people of northern European ancestry is that having a single gene for it makes people more resistant to cholera (not tuberculosis like the original said). And of course, a single gene for sickle cell disease confers resistance to malaria, which is why it is prevalent in people of African and Mediterranean descent.