Ichthyophthirius is a genus of ciliated protozoan parasites that primarily infect freshwater fish. Commonly known as "ich," these organisms cause a disease called ichthyophthiriasis, which is characterized by white cysts on the skin, gills, and fins of affected fish. This condition can lead to severe health issues and even death if not treated promptly. The life cycle of Ichthyophthirius includes several stages, starting with free-swimming infective forms called tomites. These tomites attach to fish, where they develop into cysts. Treatment often involves raising water temperature or using medications to eliminate the parasites and help infected fish recover.