Lymphocystis is a viral disease that primarily affects fish, particularly those in aquaculture and ornamental settings. It is caused by the Lymphocystis disease virus, which leads to the formation of large, white, wart-like growths on the skin, fins, and gills of infected fish. These growths are caused by the proliferation of lymphocytes, a type of white blood cell, and can impair the fish's ability to swim and breathe. The disease is typically not fatal, but it can weaken fish and make them more susceptible to secondary infections. Lymphocystis is often associated with poor water quality and stress in fish populations. Management includes improving environmental conditions and reducing stressors to help prevent outbreaks.