Lymphocystis virus is a type of virus that primarily affects fish, particularly species like tilapia and trout. It causes a disease known as lymphocystis, which is characterized by the formation of benign tumors or cysts on the skin and fins of infected fish. The virus is typically transmitted through water and can spread rapidly in crowded or stressed environments. Infected fish may show signs of illness, but many remain asymptomatic. While lymphocystis is not usually fatal, it can lead to secondary infections and affect the overall health of the fish. Management of the virus involves maintaining good water quality and reducing stress in fish populations.