How old are your chickens?
Usually chicken will build resistance to the strains of Coccidia found in their environment, so "most" of the time they do not become infected to the point where there's an overload.
However, during times of stress, illness, injury, etc. a chicken can have an overload.
You can't stop them from "spreading it". Coccidiosis is caused by Coccidia which is a protozoa found in soil and in poop. Again, once birds are exposed to the strains they will normally encounter in their environment, they build resistance.
If you've treated them and they have recovered, then call it good if they all seem happy, healthy, active, eating/drinking.