We have not established if we were talking about MG or MS. I am going to comment on MG.
I agree with the comments that maintaining a MG free flock is a tall task, though it is possible. If you start MG free, you could possibly maintain that status for long periods of time. I will let whoever decides they are interested in doing this, do their own research. Do understand that it would be a constant and every day concern. I see no joy in that.
I smiled at the comments that suggested full disclosure. That would mean that nearly every flock owner in America would be disclosing that their birds are carriers. It is that common. Just because our birds are not clinically ill does not mean that they are not a carrier for MG. If I had birds that were chronically ill (it is a chronic illness), I would not be sharing them with anyone. I would not keep them myself.
Many of us do not realize how common it is because our birds do not get sick. That does not mean that they are not producing antibodies against the mycoplasma organism.
Mycoplasma like bacteria and viruses vary by strain, one more virulent than another. Your own birds may be more resistant to one strain than they would another.
If carriers of a strain that they are more susceptible to, and are stressed, their immune systems could possibly be impaired enough for the bird to become clinically ill. It is not unusual for this same bird to have a moderate to heavy worm load, have lice or mites, and be hit by a sudden cold spell or heat wave etc. etc.
It is also possible for birds to be exposed to a strain they have less resistance to. Usually one or two or three might get sick. Commercial strains, in my opinion, often show less resistance. In twenty plus years of breeding some class of poultry or another, the only case I had of evident MG, was a commercial strain. Wild birds was the most likely route of transmission, and they were highly susceptible. I killed them all and started over. Who wants birds that we have to keep locked away to keep them healthy and productive?
If we are interested in maintaining a breeder flock, kill every bird that gets sick. That is a simple enough management protocol, and the one that I follow. I do not treat. I do not manage a hospital. I will not perpetuate weakness and I expect a reasonable tolerance (or resistance) to MG. MG is everywhere, it is common, and it is here to stay. I have not had a sick bird in a decade. That is not because
Tractor Supply is my pharmacy.
If we start right, provide plenty of fresh air, allow them to enjoy warm sunshine, young and tender greens, a good ration, clean water, room to roam, worms managed, kept louse and mite free, avoiding the careless exchange between flocks, and practice good culling practices, we should enjoy breeding a flock that is mostly problem free. Barring any unfortunate events ... like a large flock of avian flu carrying geese showing up.
In a breeder flock there are more important infectious concerns than MG.