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Yes- mortality is generally from secondary infections, but the bird can regain health and not appear to have any illness, subsequently infecting anything it comes with in breathing space of and possibly killing them. Further, most people get it by bringing a healthy looking bird into their flock AFTER any lengthy quarantine.
Unless you put a sacrificial bird in with a new bird, you can't know if it will infect your flock, no matter how long you set it aside!
Not all birds will remain carriers, and I (infectious coryza) is not passed through the egg. You have to go to incredible lengths and for a year you can't add any new birds. After a year, you can add, but you have to rish one and let it be exposed to the possible carriers. It's impossible to have a bird get it and remain in a flock without ALL of them getting it or becoming carriers.
This is a Typhoid Mary kind of illness, and you will generally have people tell you to cull your whole flock and start over. You can clear up all the survivors and vaccinate all incoming chicks at 3 weeks and 8 weeks to bring new birds in, but you can't let them get near each other and you have to be 'clean' when you go near the chicks for those 8 weeks- no germs from your carriers.
I'm so sorry! It's a horrible thing to have to deal with, but if you don't mind having a closed flock, you may make it through relatively unscathed if you get the meds to pull them through the first weeks.
Best of luck!!