I let mine roam from the very day they hatch, if they so choose, but I raise mine under a mother hen which helps them develop immunity. I would guess yours are reared artificially. In this case, since I'm keen on natural remedies, I'd advise you to start feeding them minced raw garlic with their feeds. You could combine this with plain unflavored and unsweetened yogurt or something else if they won't eat it straight.
Garlic is full of natural antibiotics (30+) which can kill bacteria and viruses that even the strongest man made antibiotics can't, because in garlic it is dynamic rather than static, so things can't get immune to it like they do to an unchanging antibiotic. When garlic is raw and freshly crushed or cut it produces Allicin from the enzyme interaction and this is quite a powerful antibiotic. Garlic also has natural sulfur, which is beneficial for the animal but will harm all parasites both internal and external, and bad bacteria, viruses, fungi, etc.
Garlic is a specific to cure and prevent coccidiosis. I always feed mine raw garlic from their first day onwards and have never seen a single case of coccidiosis, over hundreds and hundreds of chickens raised free ranging and in a mixed flock, from all sorts of genetics and backgrounds. My turkeys never had it either.
If your run has had sick birds in it before I advise you to sprinkle it with agricultural hydrated lime, and water it through the soil, as this will kill parasite eggs and many diseases. I'd do this before putting them in. Some folks get good results with their incubator/brooder raised chicks by bringing in a handful or two of dirt from the run in a tray for them to dig in, so they can gradually work on immunity, before they're introduced to the run a week or two later. Avoiding overdose of bacteria in an unprepared gut is the idea there, and I'm pretty sure your run is probably seething with it.