Koi Herpes Virus (KHV)
KHV Is a very contagious, damaging and deadly disease for koi, many times causing up to 90% loss in just a few days. Symptoms can include respiratory distress, hyperactivity, loss of coordination, and severe gill necrosis. It's usually seen when the water temperature is between 70 and 80 degrees. As with most viral diseases there is no known cure. Some people have had success with heat treatment to 86F, but the carrier state of the survivors is currently unknown.
Goldfish are unaffected.