I agree it sounded like columnaris because fungus generally does not spread that quickly, and columnaris, though generally CALLED fungus, is actually a bacteria, so you must treat with an antibiotic. The problem with antibiotics is that they can sometimes kill your biological bacteria either partially or fully (even if the medicine claims not to I have had it happen before) so it is best to treat in a quarantine tank rather than your main system (this actually goes for any medication but is imperative for antibiotics).
Columnaris is nasty and even with full treatment can take a fish. I have had more fish die with columnaris than recover, usually because columnaris shows up when the fish is already highly stressed, either from injury or another illness. I have seen one spot show up and overnight even with treatment cover the whole body.
If your other fish are healthy columnaris will USUALLY not spread from the infected fish to your other fish because the other fishes immune systems will fight it off.