From the comments of the others on this point, I see that there is an ambiguity that revolves around the meaning of the word "frequency." The origninal statement, as I read it, meant "the number of mutations per capita." However, one could read it as meaning "nuber of mutations per unit time," which would not be what the author meant.
In demographics and epidemiology, "frequency" almost always means "number of cases divided by the total population."
I have avoided using the word 'frequency' in my comments on this thread. I also think that it might be a case of the immune systems generally being weakened in those who succombed to the plague, and this is not necessarily genetics at play. There also are those with stronger immune systems who survived the plague, perhaps being exposed to the virus without becoming ill. We know this happens with many diseases today, and it is not necessarily conditional on 'vaccination'. Is this genetics?
If it is genetics, I might suggest that a strong immune system was the 'original' condition in humans, and that the weakened immune system is the 'mutation'.
Today our immune systems are weakened by many factors. Vaccines tend to have an adverse effect to the overall health of the immune system. Antibiotics are now working against us by creating resistant strains of bacteria and viruses. Our overall health is also a factor. This can relate to such basic things as dietary habits and environment.
There are also naturally occurring substances that can be used to strengthen our overall immune quotient. I believe that we can do some things to consciously strength our immune systems, even to the point of dealing with these 'new' or 'more potent' microbes that may be on the attack.
I also believe that in some cases our immune systems can be 'educated' to attack and eliminate serious diseases that are already at work in our bodies - such things as cardiovascular disease and other disorders of imflamatory origin, even cancer. However, when we do this by 'educating' our immune system, we are not changing our genetics.