I would think that being in a host and surviving is was causes mutations. A pathogen reacts to or survives it's environment and if is survives, it becomes resistant.
So, following that logic, the fewer people who come down with the illness, the more manageable it is. One way to prevent people from coming down with it is through vaccination. Fewer sick people to spread it. What kinds of pertussis mutations might exist if we hadn't treated it with vaccines?
Antibiotics....
How much of the problem with antibiotics is that people don't FINISH the prescription but stop while the bacteria is still alive and the bacteria carries on that immunity because it survived.
You can’t vaccinate against every known mutant of a particular disease unless you include every single (relevant) mutant in your vaccine.
They evolve just like we do.
I’ve seen mutagenesis in genomic data. HIV is scarily efficient with the spaghetti against the wall technique.
With antibiotics it works both ways. Every colony of bacteria has a genomic distribution of mutants within it. Every single time they multiply there’s a shot at ‘genetic variance’ of some sort. Invariably there will be some that are just not susceptible to your particular antibiotic. USUALLY there’s a reason that particular version isn’t the dominant one. Maybe the dominant one suppresses it to some extent by greater efficiency in reproducing. Maybe the dominant one secretes something that the suppressed one does not that enables it to be more successful. Whatever. Once you treat that colony with antibiotics you remove the dominant (antibiotic susceptible) strain from contention. Now, the strain that was immune to your antibiotics is ‘free like the wind’ to itself mutate into forms that will be more successful. Some of which maintain that particular antibiotic resistance. This still happens even IF you take the whole dose of antibiotics.
But it gets better. Bacteria love to chat and gossip. That antibiotic resistance portion can be ‘chatted’ amongst various other bacteria strains. And now they all ‘know the secret’.
This is why putting antibiotics into animal feed is a bad idea for a long term strategy. Unless you like those particular antibiotic resistant mutants. And have devoted a lot of research to new and improved antibiotics. We have not however devoted this research. We may alter that in the future though, when the last antibiotics stop working.
The ‘childhood diseases’ aren’t quite as genomically constrained as the pandemic ones. For a variety of reasons. One of which being that the pandemics tend to ‘burn out’ because they ARE so virulent. They just aren’t around long enough in the host to do many generations of mutation before the host dies.
The extreme genetic variability and change is why the flu vaccine needs to be repeated yearly. And why HIV vaccine hasn’t happened yet.
The issue is the ‘if you even miss ONE strain’ it can mutate and become problematic. Every generation will be subtly genomically different than the previous. Evolution happens. And when you play evolutionary games vs. something that has multiple generations per day you will lose. Every. Single. Time.