Ichthyophthyriosis is a disease of many freshwater fish caused by ciliated infusoria — Ichthyophthirius multifiliis. Affects the skin, gills of fish. Cases of mass death of fish are not uncommon.

The disease is especially dangerous in confined spaces, where it spreads rapidly from one fish to another. "Ichthyosis" is a disease that causes the majority of aquarium fish to die, can cause noticeable damage to aquatic culture. The marine "ichthyic" is a consequence of the activity of another ciliated infusoria, Cryptocaryon.

Infusoria "ichthyophthyrius" is the most common of the protozoa that parasitize fish. Adult infusoria vary in shape from oval to circle and in size from 0,5 to 1 mm, are covered with bristles and contain a nucleus in the shape of a horse's hoof.

Ichthyophthyroid invasion is usually noticeable in the form of white dots on the side and fins of the fish. Fish can also behave unusually, for example, rubbing against various objects. These white dots are pockets of fish epithelium containing ichthyophthyrius cells called trophozoites or trophonts, which feed on host tissues and can grow up to 1 mm in diameter.

After about one week of parasitism, the adult trophozoite leaves the host, settles to the bottom and secretes a cyst. A cell in the cyst, called a tomont, undergoes rapid division within about 24 hours and produces 600-1000 daughter cells called tomites. As soon as they mature, they leave the cyst and develop into a theron (vagrant), which is very mobile. Theronts then infect new fish by burrowing into the outer parts, under the scales or, most often, into the gill plates. The full development cycle usually takes from 7 to 10 days.