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KHV is not the only threat. Your home fish probably carry background levels of flukes, ick, various bacteria and other parasites. They have developed an immunity to their particular strain of flukes, ick, bacteria and other parasites and the fish can live long healthy lives despite their presence.
Bring in a strange fish and everything changes. The new fish has little immunity to the parasites in your system because the stranger has not been exposed to that strain (it is naive) and its immune system has been ravaged by the stress of shipping and handling. Likewise, the stranger is bringing in parasite strains which your home fish have never been exposed to. Someone may get sick.
There is also the dose response phenomenon. Fish which are normally immune to parasites occurring in small numbers can become sick when the number of parasites in their environment suddenly escalates. If one fish gets sick, then they may all get sick. The escalating dose comes from the stranger with its weakened and naive immune system. In this way, a stranger which is relatively parasite free when it arrives at your house can cause established fish to become sick.
Best to QT the stranger. As it's immune system recovers, start adding water from the main pond and watch as it becomes adjusted to the new microbes.
Your QT system should be as good or better than your main pond.
-steveho p
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