What is Ichthyophthirius Multifiliis?

Ichthyophthirius multifiliis is a species of ciliate protozoan which parasitizes Freshwater Fish; the disease it causes is usually called ich or white spot. It is probably the most common aquarium fish parasite and there are few Aquarists that have not met it on one or more occasions[1]. "Ich" (the colloquially abbreviated form of the name) is the largest known parasitic protozoan found on fishes. Adult organisms are oval or round and measure 0.5 to 1.0 mm in size. The adult is uniformly ciliated and contains a horseshoe-shaped nucleus which can be seen in older individuals. The disease becomes especially serious in enclosed areas, where it spreads quickly from one fish to another. Ich is the disease responsible for the most fatalities in freshwater aquarium fish and can cause notable damage to aquaculture. Marine ich is caused by a different ciliate, Cryptocaryon. After approximately one week of parasitism, mature trophozoites leave their host, settle to a substrate and secrete a cyst. The encysted cell, called a tomont, undergoes rapid division over approximately twenty-four hours to produce 600-1000 daughter cells called tomites. Once these reach maturity, they exit the cyst and develop into a theront stage, which is highly mobile. Theronts then infect new fish, digging their way into exposed parts, under the scales, or more commonly into its gill plate. The entire life-cycle takes about seven to ten days to complete.

Source: http://en.wikipedia.org/wiki/Ichthyophthirius_multifiliis