This does bring up a good point though. Do not medicate animals in quarantine. If they are sick, you want them to show symptoms so you can see if they're a danger to your birds. I know you were just trying to be compassionate to a bird that needed some TLC because you're a caring and nurturing...
I won't have to add new blood for ages. When I do, I get hatching eggs from the most reputable breeders I know and have known for a long time. I do not buy started birds and should never have to.
A word about quarantines....
It's not a foolproof method. I did quarantine the birds that brought MG to my flock. The thing about MG is that infected birds, once they are over the initial symptoms, become carriers. So they will spread the disease without showing signs of illness themselves...
And were your chickies suffering, so you decided to finally cull? Why didn't you choose to treat them, was it already to late? Are roos not affected by MG?
All the birds are affected by MG. You cannot get rid of MG by treating it. It stays in their bodies forever, which is what "carrier"...