I overheard a couple of docs talking recently about mycoplasma being absolutely everywhere in the environment, so I think that we are lucky that we all don't have something like this going on in our flocks.
Yep. Most poultry veterinarians estimate 80-95% of backyard flocks are infected with MG. It appears when birds are stressed: heat, cold, shows, new birds, laying...something. I am a vet, and I had one bird get what looked like MG. I debated treating versus culling. But it was my best bird, so I treated. He died, but nobody else got sick. It happens.