Starting in the late 80s, rainbow trout in Colorado began dying off because of a parasite that causes whirling disease. By 1997, wild rainbows in the state had all but vanished. The disease is caused by a water-borne parasite that infects young trout and some species of salmon, causing deformities of the skull and spinal column. The infected fish swim in circles, hence the name whirling disease. Ultimately, it leads to death.