The causal agent of flexibacteriosis is generally thought to be a Flexibacter sp. However, these are part of a wider group that is difficult to identify and characterise. This group is comprised of several genera including Cytophaga, Flavobacterium and Myxobacterium. They are usually chromogenic, filamentous bacteria that can cause disease conditions as diverse as columnaris, fin rot, coldwater disease and gill disease. These conditions are capable of causing severe losses in susceptible fish populations, particularly at low temperatures.