Unless you keep your chicks in a completely sterile environment, they will be exposed to coccidia. It's a parasitic protozoa that can dwell in soil. It's only a becomes a problem when the protozoa grow to numerous in the chick's stomach and intestine. Chickens gradually become resistant to the strains that they are exposed to. By 3 weeks old, they should be getting short trips outside to play. It will help them to gradually build up resistance. While they are young chicks, they are extremely susceptible to developing Coccidiosis. If they aren't going to be on medicated chick feed, you need to have something ready to treat them with just in case.
