This does not appear to be an illness. Either you have a predator getting in or your birds are cannibalising each other. How big is their coop (how wide by how long in feet) and how many chickens do you have? Lack of space, particularly if the weather has been bad and they have been confined to the coop will cause they to get stressed and start pecking each other and once blood is drawn, they sometimes all pile in and start attacking.
Is the coop and run predator proof? Small predators like stoats can get through as little as an inch and a half diameter hole. Where in the world are you? (It helps if you include this info on your profile page.... nearest city and country is enough) There are different predators in different parts of the world. If you are in a warm climate, then you will need to be careful that flies don't lay their eggs in those wounds and eat the chickens alive.