While columnaris is ubiquitous in all natural environments (ponds etc), it can be kept out of the aquarium environment by properly quarantining all new inhabitants, (I do this for at least a couple months or more). If infected, the stress of transport, new water and new digs, will usually bring it on in an infected individual quite quickly.
And to me, euthanizing that new fish, or group is a very reasonable alternative to what happens next, if not done. (time, the stress to the aquarist, and expense of treatment, especially if it spreads to an entire fish room)
If the fish is a asymptomatic reservoir and carrier well "Everything is F...ked" Walter sobchak, the Big Lewbowski