Coccidiosis can get real serious. The problem is not that there are some coccidiosis bugs in their system, it’s that the numbers get out of hand. That bug can thrive in warm wet manure like on a wet coop, run, or brooder floor. If you can keep it pretty dry you can really help yourself but that does not always work. Some strains of that bug are pretty strong. Still keep the water clean and the coop and run as dry as you can. If you do have an outbreak, get Corid and treat according to the directions.
I don’t recommend Corid as a regular preventative. You want enough of those bugs to reproduce so they can develop immunity, but if you have an outbreak, use it.
I don’t know a lot about your situation, but if they have been on the ground or exposed to the Coccidiosis bug for three weeks or more, they should pretty much have immunity. The most dangerous time is a few weeks after they are first exposed to the bug.
Immunity to one strain does not give immunity to other strains. Have you brought in any new chickens lately? That’s a real common way to bring in a new strain of Coccidiosis. A wild bird could have flown overhead and dropped a packet containing a new strain or you may have tracked it in on your shoes just from dirt. That stuff spreads really easily.
How sure are you that it is Coccidiosis? Could it possibly be something else? Corid will only treat coccidiosis. It’s not an antibiotic. It will not treat anything else.