"EHD is a viral disease and the most common whitetail disease. It’s transmitted by biting midges, commonly called no-see-ums, and it happens every year.

During the first few days, the infected deer might look and act normal or possibly show minor signs of illness. After the first week symptoms become more apparent and can include depression, fever, swelling in the head, neck, tongue or eyelids, difficulty breathing, loss of appetite and the deer could loose its fear of man.

Then it gets worse. Some deer develop ulcers on their tongue or have the thick pad on the roof of their mouth begin to erode. To make matters worse, fluid can build up in the lungs and the lining of the rumen can scar. As the symptoms worsen fever sets in and the deer seek out water – that’s why infected deer are usually found dead around water sources."

http://www.outdoorlife.com/blogs/big...ease-explained