Curious if they are in a confined place... do they free range ?
I only ask because obviously it is contagious and therefore can stay on the ground to contaminate other birds or re-contaminate others. I wouldn't freerange till you know what the Roos had.
Personally, depending on the disease, I might just cull the whole flock if they were in a confined area... make a NEW coop/run area FAR away from the other one (up hill and upwind) and start again with clean chicks. yes it is a long process but to be Self Sufficient you want the BEST for you and the Family
Culling is not a bad thing--- keeping a sick or injured bird alive is, in my book.