I'm no expert on EHD but have done some reading on the topic. As Semi states, it transmitted by a vector or some kind of insect. In our world the vector is a gnat we call " no see'em's " because they are so small you can't see them.

The virus is in the head every year but needs to be transmitted from deer to deer. Then you have the whole immune theory discussed above. When food and water are plentiful, the heard is scattered and the opportunity for deer to deer transmission is decreased. When deer are concentrated at water source from a drought or at a feeder when food is tough, we have problems.

The wet spring Semi talks of helps build the gnat numbers, then a dry summer pushes the deer to a water source and you have a bad combination.