I would probably treat everyone since Corid is not harmful to chickens. Older chickens are usually immune to what's on the farm already, unless new girls show up with new strains, or somebody tracks a new strain in on their shoes. Any new hens should be treated when they arrive since you have...
I would probably treat for coccidiosis since you have had multiple deaths. Corid liquid (from the cattle aisle at the feed store) 2 teaspoonful per gallon of water given to all chickens for 5-7 days is the best treatment. Give no vitamins or vinegar in the water until after the treatment, then...