Coccidiosis is a disease of protozoa in the gut of the chickens and in the soil. They usually develop immunity to it by 5 months old, but it can cause death or damage to the intestines. Chickens who survive a bout of cocci may never gain full weight or lay as well as others. If one chick has it the other chicks are at risk since it is very contagious. Diatomataceous earth or DE doesn't help or prevent cocci. Corid is the best treatment, but if you have Sulmet, it will treat the 2 worst strains of cocci.