I also just had an outbreak of columnaris and I'm not too sure where it came from. I had added 4 kuhli loaches about a month before the outbreak. They had been quarantined for two weeks before that, and have never shown any symptoms of columnaris.
I also added a number of plants, snails, and some shrimp I got from AZ Gardens. They also had been quarantined, but only for about 4 days (I mostly just wanted to make sure no ich hitched a ride.
The only other cause I could think of was recently doing a gravel vac of my Aquasoil. The outbreak started within a week of doing this. It had never been cleaned at all (even skimming over) for the year I've had the tank. Maybe this stirred something up. I read somewhere that columnaris is actually in most tanks, living on dead matter. Perhaps my cleaning the tank suddenly put a lot of it in the water column, and it was more than the immune system of some fish could handle.
Fortunately it has not been all that virulent. I have a least a dozen mollies and about 10 other occupants (5 other unrelated species), and only 3 mollies and one angel ever got sick.
It took a while before I figured out what it was even going on. I thought it was just a fungus on two mollies. I moved them to a 5g bucket and added Jungle Fungus clear. This seemed to clear things up (it actually is a recommend treatment for columnaris). During this time the angel sometimes had a small white pimple or blotch, but it came and went, and I figured if it was just fungus, and would go away with clean water.
A few days after I put the two treated mollies back in the main tank they were sick again, and now the angel was acting a bit funny (hiding during the day unless it was feeding time). He probably had had columnaris well over a week by then, but still not showing much more than a small white blotch on his side. However, symptoms on the mollies were more clear this time, one being completely covered in white tufts or gray blotches. At this time I finally realized it was columnaris.
I moved the 2 sick mollies, a 3rd molly that got sick, and the angel to a 10g tank to treat them. This time I used Furan-2 (as is often recommended), and only afterwords found out it has the same ingredients as Jungle Fungus Clear. I also added melafix, although I think pimafix is suppose to be a better choice for columnaris (should arrive in the mail tomorrow). I also decided to treat the main tank, since I figured at least one of the mollies probably had it, and some may have it without showing symptoms.
It's been 5 days since this last round of treatment. No new cases in the main tank. The angel still has the white blotch, although it is very small now. He acts pretty normal. The mollies are clear of while blotches, but show a bit of fin damage, and tend to hide near or behind the HOB filter most of the time. They were also treated with a Potassium Permangenate bath twice a day for the 1st 3 days (but not the angel, he's too big). All are eating well.
I have my doubts that Furan-2 is going to lick this, since my Fungus Clear treatment (same ingredients) may not have completely cleared it up in the treated fish, so I may now have a more resistant strain. I'll continue the current treatment for the full 8 days and then see what happens. If it comes back, I may go the Triple Sulfate route next time.