Ichthyophthirius multifiliis is a single-celled parasite commonly known as "ich" or "white spot disease." It primarily affects freshwater fish, causing white cysts to form on their skin, gills, and fins. This organism thrives in warm water and can spread rapidly in aquariums and fish farms, leading to significant fish mortality if not treated promptly. The life cycle of Ichthyophthirius multifiliis includes a free-swimming stage that infects fish and a cyst stage that develops on the host. Treatment often involves raising water temperature and using medications like formalin or copper-based treatments to eliminate the parasite. Proper tank maintenance can help prevent outbreaks.