If the coryza birds are still alive and on your proeperty, it would be difficult to have 100% biosecurity against spreading the disease. They still are carriers, and germs can spread on hands, shoes, clothing, and in the air. Coryza and MG are only alive for several days after the chickens are gone. It is best when a flock gets a disease like coryza, to close the flock and not add new birds until those ar all gone eventually. Sorry that you are dealing with a disease. Getting it identified through testing a couple of birds or having a necropsy done by the state vet on a sick bird would be helpful.