Whirling disease is the common name for an infection in salmonids caused by the protozoan, Myxobolus cerebralis. Diseased fish usually show signs of circular swimming, hence the disease name “whirling.” In addition, diseased fish may show other signs, such as black tail, skeletal deformities, and shortened gill cover. Because of the erratic, uncontrolled circular swimming, the fish are unable to eat or escape predators. Myxobolus cerebralis has a two-host life cycle, alternating between salmonid fish species and a benthic organism, the worm, Tubifex tubifex. The worms live in the mud of streams. As many as 10,000 worms can be found in one square yard of a muddy river bottom.