chicki,
Uronema reproduce asexually. This means by binary fission, so it only takes one Uronema to make many. From my experience one of two conditions have to be present to have the population of Uronema explode in your tank, which results in them attacking your fish. One, a more than normal accumulation of solid organic waste ( uneaten food, dead organism underneath a rock ect...). Secondly, damage to your fish from another organism ( fish on fish aggression, fungal disease ect..). Hope this helps.
Brad