It is generally believed that fish diseases are a product of the hatcheries and that they rarely occur among wild fishes. This impression is largely due to the difficulty of detecting and studying diseases in fish under natural conditions. There is good evidence that infectious diseases are quite common among fishes in nature and that they exact a heavy toll each year. The most destructive diseases are caused by bacterial and protozoan infections. Virus diseases may result in widespread epidemics, but the mortality is usually not high. Infections with fungi are not uncommon and may cause considerable mortality. Parasitic worms and copepods, while common, appear to be relatively less harmful.