from what i know, yes its one of those things that is nearly always in your tank, and only takes hold when the fish are stressed. I think the most common cause of it is bad water quality/overstocking. There's lots of different treatments, like raising the temperature, salt, lots of water changes, different meds, but i've always just used wundercure and it goes in a few days. I've had a few batches of fish from shops come with it but its never been a major problem.