The big cause of EHD is drought. Its a virus is spread by little gnats(called a midge) in stagnant water holes and is killed off every year by the freeze assuming you live in a climate that freezes.....I don't know what happens if you don't get cold weather.

Drought concentrates deer into around stagnant water where these little gnats thrive...It can happen to any size herd. Of course the more deer you have the more it will kill but other than that its about the population of these gnats not the population of deer.