My fish, which I know came from the same two sources as yours, were infected with whatever this is.
Someone agreed to look after my fish and put mine in with theirs, even though I advised them against it because I'd had problems with the disease.
Theirs got it and died within a week. They lost 55 fish in that week, which is abnormal for this person because they'd only lost 7 fish this year until adding my fish to the mix.
I think that's about as scientific as you're going to get from me, but if you read the article I linked, the writer managed to infect his healthy fish simply using water from an infected tank.
I'm not saying there is no genetic component to it. I'm sure there is. I think that everything Pegasus said here is probably true, but that unlike MS, there's some sort of infectious component that sets things off. I just have no idea what kind of infectious agent it would be. When the onset of the disease began in my tank, I had not added a new fish, any new plants or snails, or anything but water for over 3 months.