From what I understand, EHD causes the deer to bleed internally, which is why they are usually found near water when they die... The loss of blood causes extreme thirst. In regards to it hitting bucks the hardest, I've also read that antlers in velvet being full of blood are like a huge magnet for the gnats (midges). Bucks tend to lay pretty low when their antlers are in velvet and tend to stay in the low, damp areas were the midges are...they don't move around as much as the does and fawns.