Bacterial cold water disease (BCWD) is a bacterial disease of freshwater fish, specifically salmonid fish. It is caused by the bacterium Flavobacterium psychrophilum, a psychrophilic, gram-negative rod-shaped bacterium of the family Flavobacteriaceae. This bacterium is found in fresh waters with the optimal growth temperature below 13 °C, and it can be seen in any area with water temperatures consistently below 15 °C. Salmon are the most commonly affected species. This disease is not zoonotic.