The cocci are in the ground, just about everywhere. It is a parasite. Most mature chickens have some of it, but are resistant to it. The new, young chicks are not, and can be overwhelmed by the parasite and will die very quickly. It's in the ground - passed on by other chickens who are resistant, and the youngsters pick it up. Amprolium is a thiamine blocker and helps the chicks develop a resistance UPON INTRODUCTION TO THE PARASITE. So it is best to continue the medicated feed for at least 2 - 3 weeks when the chicks go from the brooder to the real world outside. It does no good to stop the medication as they go outside, because they need to encounter it to be "innoculated" and develop resistance.
Some locations are natually high in cocci and it's best for owners to prepare their chicks for encountering it.
Momma hen hatching chicks introduces it to her chicks through her own feces. So broody-raised chicks don't need medicated feed. They MAY need treatment, as well as some older birds, too, if it takes off in a flock - that's what the Corrid-V is for: active cases.