I've seen lately where several people have bought or adopted birds and just put them in with the flock with no quarantine period to check for disease (note, check for it, not treat for it). Please read this thread again to refresh your memories. There is another one with MissPrissy as the author-search for that one; couldn't locate it at the moment.
https://www.backyardchickens.com/forum/viewtopic.php?id=12751
https://www.backyardchickens.com/forum/viewtopic.php?id=12751