There are a lot of different herpes viruses in the world, including one that affects snakes. They're not an STD in many species--in fact, humans also are afflicted by a strain that isn't an STD. Herpes simplex virus type 1 is the most common cause of 'cold sores'--transmitted by contact, so a kiss will pass it on.
Since these fish are in the water, they need only be near one another to transmit the Koi herpes virus. The mortality rate is quite dramatic, since the virus attacks the gill tissue.