The oldest CF mutation seems to be about 56,000 years old ~ and that's the time when the White Folk went West from Central Asia and those who were to turn into Chinese went East from Central Asia.
The Cro Magnon's who went to Europe walked right into an interstadial where RATS and other rodents were cock of the walk ~ and the black death is rodent born, in part, by fleas that live on those rodents.
Humans exist in large numbers in Europe only by having an immunity to rat born diseases ~ CF may well be the gene that confers that immunity.
In between the great plagues humans prosper, more and more of those with no CF gene mutations survive, and next thing you know there's another great plague (usually the same old same old ~ the bacterium, Yersinia pestis)
Still, the underlying assumption being that the next pandemic will be plague/bacterial-based.
If a Spanish flu-type virus does a number, that entire evolved immunity becomes hopeless.