Behavioral fever, something newly defined in the behavior of koi/carp.  What a great article on the study of the Cyprinid Herpesvirus3 (CYHV-3), also known as KHVD, and how water up to 90 degree F (32C) is used for treatment.  Though a vaccine is still needed, this study showed that fish will self-select this high temperature water to cure themselves.  "It's been known for a while that fish (and other animals that can't generate heat internally) could help their immune systems fight off infection by moving to a warmer spot. The phenomenon is called "behavioral fever."
 
Each year, fish farms produce a massive amount of carp — so much that if you put all that fish on one side of a scale, and all the people living in the U.S. on the other side, they'd pretty much balance each other out by weight.
But for the past couple of decades, carp have been plagued by a type of herpes virus, known as Koi herpesvirus.
Now, as researchers report this week in the journal Cell Host & Microbe, there's a simple way to prevent fish from dying of the virus. And during their investigation, the researchers also found something intriguing about how fish — and humans — fight infection.
 
https://www.npr.org/sections/thesalt/2017/02/09/514107534/in-studying-si...
The study: https://wwwnc.cdc.gov/eid/article/16/12/10-0593_article