Oh, Pink, do not listen to such.
I can absolutely guarantee you
they get sick birds and do not cull their entire flock.
As I state over and over, migratory birds bring in new germs, diseases and sickness all the time. I used to bird watch a good bit and we were always recording birds that appeared to be sick when they migrated through for the spring and fall for use by the American Audubon and Bird Conservancy.
There is no way to stop it from happening unless they do like the big poultry houses and totally enclose the place and suit up every time they go in because even back yard fanciers keeping them in a sealed room carry it on their shoes. Rather ridiculous, don't you think?
And these are the same people that go to poultry shows and put their birds beside and behind other chickens not knowing what they may be carrying that they are not tested for, like any upper respiratory infection.
Having worked in the animal husbandry field most all of my life I have seen people spout nonsense that they themselves do not live by.
I say get him better and enjoy your flock. Your birds can not live in bubble all their lives. Wild birds poop into runs, sneeze over water dishes and leave feathers with follicles attached floating around the yards.
I am all about natural immunity as best as I can manage it. We just can't afford to have our flocks in a controlled environment to safe keep them like the big poultry houses.
edited for name-calling