From my experience I've learned they need to be quarentined, and not just in separate pens on the other side of the yard.
I'll be honest with you, which my experience may be a somewhat rare occurance I'm not sure, I will NEVER buy adult chickens again.
I don't know I'm just so sick over the whole situation and I'm hating myself for not being educated enough going into it to bring disease into my flock.
I have 5 that look like they will be dead within the next few hours and all the rest are really sick. Waiting for antibiotics to kick in but nothing.
Honestly if I lose them all,I think it will be awhile before starting with new chickens. My chickens are all pets and I was super close to a few they were like family members.
If you do get adult birds, just make sure they aren't anywhere around your existing birds for a month.
This is the hardest lesson I've ever learned and all I feel like doing right now is lying in bed and crying

Praying that no one else goes through this, and hopefully this extreme loss will enlighten someone else new to adding chickens like I was and avoid another family and flock from the heartbreak.