Bacteria, viruses, and parasites all cure more quickly when the koi's immune system is functioning properly. In essence, this is what happens when people get a fever, it is the warm blooded animals natural response to activate the immune system. Koi, being cold blooded, do not have this advantage. We have maintained an insulated room with 8 warm tanks at Laguna Koi Ponds for three years now. Most Koi respond very positively to the warmer water.
Viral diseases are only treatable by the fishes natural defenses.
There are no effective medications. The most severe cases do receive antibiotics incorporated into the food. Koi which receive too much pellet food in the winter can become congested and acquire bacterial dropsy symptoms. Food is not being digested at low temperatures and begins to ferment. Heat therapy gets the digestive enzymes working.