LaFleche is right...coccidia is everywhere and you can't sterilize the soil or anything else against it; you can only treat it as chickens grow up enough to develop resistance.
I should add I didn't mean to imply that LaFleche's answer was incorrect, or not good advice! I do believe that sulfadimethoxine is more effective than Corid, but when it's a bad outbreak and lives are at stake, I personally prefer to kill the danged coccidia rather than just "handicap" it...
Most effective is a drug called toltrazuril which actually kills coccidia...Corid and sulfadimethoxine set it back until the chicken's immune system can catch up and fight it off. You can get toltrazuril in a powdered form called Endocox at Jedd's Bird Supply (maybe other places, but that's...