Eye infections in flock from breeder. How do I stop it from spreading

Bought this new flock from a local breeder that I never used before. One chick died the day I brought them home. The second one seemed to recover and a third one only had one eye (that 's what it looked like at the time). These chicks are mixed ages between 3 to 4 months. Anyway, I've had them about 3 weeks now and just noticed that one was missing. Looked in the coop and found one that had been dead quit awhile. Noticed another one where her eye is swollen about 5 times it's size and I don't think it 's "one eye" she seems to be ok. Any ideas what this might be and if so how can I treat it so the rest of the flocks don't get it. Please give dosages if possible for that age group.
THANKS,
Jo