I'm a new flock owner and have 10 adult chickens (I introduced eight new ones to my two existing hens two weeks ago), and I have 20 chicks in a brooder. Last week, one hen started looking lethargic and her breathing was rattled. I isolated her and gave her VetRX. Her breathing cleared and she got more energy within three days. Since then, two other birds have developed the same symptoms. I'm treating them the same, but I'm very nervous about my flock. I've been reading that now I must have a "closed flock" and shouldn't incubate any of these currently ill chicken's eggs in the future (assuming they'll live). Do I have to keep these upcoming chicks separate from my existing flock once they mature? I certainly don't want to continue treating new birds, but I also don't want to cull any of my existing flock members if they can overcome their illness. Any help would VERY MUCH be appreciated!!