Well, they won't all get it, though some might. You might not have any others get it. It's just the luck of the draw, I think. If you can get some Oxine (not the kind with crystals) mix with water in a spray bottle and spray everything, including misting the birds themselves, and you can put a few drops in their drinking water. You can do that with Oxine, but not bleach. Oxine is pretty pricey, but a little goes a long way, and it's awesome disinfectant. If you can't get it though, wash all the feeders and waterers with bleach/ water solution. And pray. If you do have another one start showing those symptoms, start them on Hypericum Perforatum, and I mean as soon as you see they look like they have the same symptoms. Either they will get it, or they won't, and chickens (even chicks)have remarkable immune systems, for the most part. So you might not have any more worries at all. It's not worth spending a lot time fretting over, imo. Sometimes just one bird will get something and then you never see any trace of it again in the flock...for months, years, or ever. .