You can close your flock to any new birds, and no selling or giving away birds or hatching eggs, and treat your sick birds. The others may already be carriers, so you would have to cull all birds on the property, clean, and wait a few weeks before getting new birds to toally get rid of a disease...
The second chicken could have the same problem, but it also could be an injury from pecking, and infection could be a problem. Usually chickens with respiratory diseases, especially coryza, would have coughing, wheezing, nasal drainage, along with the swollen face or eye. With MG, symptoms can...
Mycoplasma ( MG,) coryza, and other respiratory diseases can cause a swollen goopy eye. E.coli and other bacteria can make it worse. Did the eye initially have a bad smell that might indicate coryza? MG is treated with Tylan and oxytetracycline, while coryza can be treated with sulfadimethoxine...