Fowl pox is a virus spread by mosquitoes. Dry pox is usually not serious, and will run it's course in 3-5 weeks. Wet pox can be more serious causing yellow patches inside the throat that can block the airway. Pox can be complicated by other infections in the eyes, for example. Some birds with pox can starve themselves, so make sure they are eating well. It would be good to see more pictures since the foam in the eye of the one chicken could be from MG or pox. MG can be treated with Tylan 50 injectable given as a shot or given orally to each chicken. Oxytetracycline is also used in the water to treat it. If it is MG, the new birds could have brought in the infection as carrier birds. You can get testing done, or get a necropsy done on a bird that dies to get a definite diagnosis. Sorry that you are dealing with this.