Coccidiosis is caused by ooccysts in the soil. When they overwhelm the gut, they cause bleeding and intestinal damage. One way to avoid some of that is to make sure they get exposure to soil from their first week of life, even if you just sprinkle it in the brooder. There is a cocci vaccination, but I've never vaccinated my chicks for anything. Even on medicated feed, they can still get cocci; it's really no big deal and easy to treat if caught early, but if not noticed, it can kill.
Chicks raised by a broody hen never seem to get cocci, probably because they ingest some of mom's poop and she is already immune, plus they get exposure much earlier than chicks raised indoors in a brooder.
Chicks raised by a broody hen never seem to get cocci, probably because they ingest some of mom's poop and she is already immune, plus they get exposure much earlier than chicks raised indoors in a brooder.