How do you major breeders do it? Keeping flocks safe from outbreaks

[Mosey2003]

I manage it by not accepting random birds. I have one breeder where I am comfortable buying birds. There is another that I would be, but she isn't selling yet. Otherwise, it doesn't come home with me. Sometimes I will take birds as a favor (trade a pullet for a cockerel), but those birds are butchered when we get home. If a bird leaves here, it doesn't come back alive generally.

I did have a problem last winter with wild birds staying in my barn and giving mine a respiratory infection. Infuriating, but what do you do? I'm not making a bird-proof enclosure.