This is not always your fault.
I have a closed flock. I'm absolutely paranoid about biosecurity because of some things I have seen and done at work. I don't wear street clothes out to care for my birds, and I don't wear farm clothes off the farm. My birds free range, but the groups have enough room that their paths rarely cross. I don't allow visitors. I never bring birds from anywhere else here. I hatched all our chickens except for the first birds, and those all came from a breeder who is careful. We haven't shown birds, and I am not sure I want to because of the quarantine needed to bring them home.
Well, I have one roo with goopy eyes and sinusitis, and he is from a group I do keep penned. None of the rest have anything, and it's been almost a week. I noticed some sneezing in the roosters we slaughtered some weeks ago, one reason I butchered them right then, in case it was something. But this one bird is now sick. I am treating him withTylan-50 (it's not working as well as I would like, so I'm starting Baytril tonight) but plan to have him tested if he dies or I euthanize him. I know what many people say about respiratory disease in birds, but I also know the statistics and the odds. I suspect it is IC.
It's a difficult decision to make. What if you had one or two tested and then decided what to do? Maybe the original bird and a chick or two? Contact your state veterinarian for information on how to have birds tested in your state. I'm sorry you are dealing with this.