Ichthyophthirius multifiliis is a single-celled parasite commonly known as "ich." It primarily affects freshwater fish, causing a disease called "ichthyophthiriasis." The parasite attaches to the fish's skin, gills, and fins, leading to symptoms like white cysts, lethargy, and difficulty breathing. The life cycle of Ichthyophthirius multifiliis includes a free-swimming stage, allowing it to spread rapidly in aquariums and ponds. Treatment often involves raising water temperature or using medications to eliminate the parasite. Proper tank maintenance and water quality can help prevent outbreaks of this disease in fish populations.