Ichthyophthiriasis (White Spot) has been reported from virtually all areas of fish cultivation and feral populations of fish from most continents. The spread of the causal agent, Ichthyophthirius multifiliis, is thought to be as a result of fish transportation. I. multifiliis is the most pathogenic protozoan parasite of fish and is considered to be a major problem in aquaculture. It appears to parasitize all freshwater fishes, since there are no records of any species with natural resistance, although there may be variation in the degree of susceptibility between fish. Disease outbreaks occur given favourable conditions such as a suitable environment and susceptible fish. Severe infections occur most commonly in dense populations of fish. Important criteria for an outbreak are warm, or increasing, water temperatures and stress. The condition is characterised by a variable number of white spots on the surface of the fish.