Let your chickens eat dirt in the woods/yard. Woods is the best. It will cure cocci and prevent it. The common disease to worry about that you can't do anything about is Mareks. That's a bridge too far once infected. That looks like a neuro disorder with the neck recessed back, lethargic, hard to keep eyes open and zero concern or motivation. Adult birds normally are less susceptible to Mareks. If they can roam the unfertilized or weed poisoned landscaping - they should not have a cocci issue at all. Let them eat dirt, good old fashioned dirt and not a bunch of chicken crap compost in an enclosed run and coop. That's not their natural environment where they thrive. Its not the same. This could also be a ton of other crap. Need to know more. Need to see the bird, and what its vent looks like. If it passes do your own necropsy and investigate the digestive tract. Tons of videos. Look at its digestive tract to see whats going on. Sorry couldn't be more help. Good luck.
