Myxobolus cerebralis
Whirling disease hurts young fish the most, and can hurt bones and nerves. Fish with the disease do not swim very well, and they "whirl" or spin instead of swimming straight.  This makes it hard for the fish to get food and get away from danger.  Dealing with M. cerebralis costs more money than most other parasites in its group (which are called "myxozoans"). People do not get sick with whirling disease, though; only fish do.