Ashley, according to the Chicken Health Handbook and online sources, this is a bacterial disease (just clarifying) so why not treat the flock with natural antibiotics? You can buy [cheap] echinacea/goldenseal capsules at walmart or any drug store. They act the same way pharmaceutical antibiotics do except they don't destroy the healthy flora of the birds' system. Also crushed Vitamin C tablets are awesome. There's so much you can do to help them. Don't give up.
What about oregano/oregano oil and garlic?