The degree of pathogenicity caused by an infectious organism, as indicated by the severity of the disease produced and its ability to invade the tissues of the host; the competence of any infectious agent to produce pathologic effects. (Bondad-Reantaso, M.G., McGladdery, S.E., East, I. & Subasinghe, R.P. (2001) Asia diagnostic guide to aquatic animal
diseases. FAO Fisheries Technical Paper, (402/2): 237p.)