If it is infectious bronchitis virus, the most common and mildest respiratory disease, there is no use to quarantine any of them. It spreads through the whole flock eventually over weeks, and does not cause the eye bubbles or swollen eyelids that MG can cause. IB is a one time disease— once they have it and recover that is it. They remain carriers for 5 mo. up to 1 year. MG and some other chronic diseases can come back again whenever they are stressed during cold weather or molting. My flock had IB once about 6 years ago apparently from wild birds since I had not added birds. I waited over a year after all recovered, and did not see it again.