Updated April 2018
Things that are scary about KHV:
If it gets into your collection, KHV can kill a lot of fish.
KHV establishes latency in Koi it has infected but not killed; a latent Koi is then called a carrier of KHV.
KHV doesn't have to cause the full blown disease in order to establish latency.
Latent Koi can start shedding the virus, which can infect that fish or others; stress can bring this on.
KHV can survive in sediment (mud, or mulm, on the pond bottom) and in filters for a long time, maybe even years.
Koi are bottom-feeders and suck at the mud; one route KHV enters a Koi is thru the gut.
Several other fish species can harbor, propagate, and shed KHV without showing clinical signs. If Koi are kept with shedding carriers, those Koi can be infected.
There is currently no good way to check easily and directly for latency, i.e., no commercially available simple test that searches for viral DNA in the Koi. However, latency can be inferred by an antibody test (this requires a blood draw...not all that easy).
There is no commercially available vaccine for KHV.
If a Koi becomes actively diseased and/or a latent carrier, there is currently no cure for these conditions.
Things we can do to try to protect ourselves (and our fish):
Consider not introducing new Koi into a "clean collection."
Buy from reputable dealers. Many dealers have had KHV in their facilities. Buy from ones who own up to it and do what's necessary to clean up their facilities.
Quarantine new Koi. Three (3) weeks at 70°F to 75°F is sufficient to observe a KHV outbreak (i.e., active disease); two (2) weeks at 75°F to 80°F is sufficient. Note: not all Koi carrying the virus will break out with the active disease in a quarantine situation. Stressing the Koi will facilitate an active breakout if the fish is carrying the virus.
You may want to consider having an antibody or viral neutralization test done to determine if a particular Koi has antibodies to KHV. A positive test that the fish has antibodies and implies it is a KHV carrier. Such testing requires a blood draw, which is not all that easy.