I myself believe its remains in the tank always and takes something to get into plague proportions
I have had a tank where I didnt add anything but water for months and then had it break out
AsFrenchy has said id say it attacks the fish in parts where it can be seen easily and only a couple of the parasites keep going through its life cycle and the fish is strong enough to get through it by itself
Then something happens and the fish loses alittle of its immune system and bang the parasite can take hold and mulitply very quickly causes it into plague proportions
Brad