Corid and Sulmet only treat coccidia if they have it. I have had full grown chickens come down with, vet did fecal. Said in our hot, humid wet conditions things develop. Also if you bring in a chicken that has a different form of coccidia then you chickens are use too, they will get that form and can get sick. Coccidia has like 9-12 different types/forms, your chickens build up tolerance to what ever type is on your ground/poo etc... Something new comes along and poos, your chickens eat it and then they have a new form they are not use too. If you have not wormed your chickens, nor brought in any new chickens , worm with the Wazine and see if they have regular worms.
If you did not see your chicken limping with just one leg, or lose control of one side of her body, then I doubt she had Mareks.
If you did not see your chicken limping with just one leg, or lose control of one side of her body, then I doubt she had Mareks.