Viral nervous necrosis (VNN) is caused by betanodaviruses, or spherical viruses that targets the nervous system and kills brain cells in fish in order to survive and thrive. When it hits, VNN causes fish to act crazy. It enlarges the swim bladder, resulting in abnormal swimming behavior such as whirling and loss of balance at the water surface. The virus produces lethargy, reduces appetite and darken skin pigmentation. Fish kills Result.

Icamina, P. M. (2011, September 9). The effects are maddening as they go viral. Malaya Business Insight, pp. A9, A10