If you look lower down on this thread you will see KHV from the SAKKS Guide. You will find much info there.
KHV is activated by stress and temperature.
First signs - your fish start dying.
Although I do believe the virulence of KHV is changing. When KHV first came to this country in the late 90's and infection would produce rapid deaths and a 80 - 90 % mortality rate.
There are many case recently where the mortality rates are 30% and, interestingly the koi do not die so rapidly. A few cases have had koi deaths over weeks and months rather than days. This leads some koi keepers to believe that it is a bacterial infection rather than KHV. In the early days of the disease deaths would happen in a few weeks.
I wonder if this is due to the virus mutating - a virus lives in the cells of the host - which is why you cannot kill it. And it if kills the host rapidly it cannot replicate its self and therefore a virus that kills rapidly (as KHV did) is self defeating.
Secondly I wonder if there is a natural immune response within the world koi population to KHV as this virus is now imbedded in the koi population.
Yes - koi can be carriers. Carriers can be survivors after an out-break and in all probability koi that have been vaccinated will themselves be immune to the virus but can still spread the infection.
Think of Typhoid Mary - in the early part of the last century this unfortunate lady contracted typhoid - she became a carrier of the disease (and was totally immune to it) and show no signs what-so-ever. She always wanted to work in kitchens! So she spread typhoid and many people died.
There is much we do not know about viruses and so far no one has found a cure for a virus infection.
Keep in mind that KHV is not the only thing that can kill your koi.
Buy from reputable dealers who quarantine new koi and test for KHV (although that is not a complete guarantee that all the fish are KHV free.) But they have made an honest effort to control the disease - which many cowboys in the industry do not do.
Hope this helps.