This is freshwater disease only, not the same as the Marine Oodinium. Cause is a parasitic dinoflagellate,viz. O.pillularis and O.Limneticum.
Not immediately fatal, normally takes a few days before casualties will occur in large numbers. However its reproductive rate is very high and it can rapidly spread.
Reasonable, if caught in time, & environmental conditions are first rate.
Dr. Jacobs, Bruno
Fish Medical sprl