Bluetongue disease is similar to but not the same as EHD (epizootic hemorrhagic disease) but the symptoms in whitetailsare the same for both and both cause high mortality. Both diseases are spread by biting gnats and are naturally occurring. The reason we see it so much in drought years is that deer that are normally somewhat spread out are now concentrated around whatever remaining water sources there are. If these water sources also happen to be in gnat infested areas where the disease isalready prevalent, you have the makings of an outbreak and people start finding a lot of dead deer. The only thing that will stop it is a killing frost in the fall to get rid of the gnats. Here's a couple links if you want to read about it.
http://www.aphis.usda.gov/lpa/pubs/fsheet_faq_notice/fs_ahbluetongue.html
http://www.michigan.gov/dnr/0,1607,7-153-10370_12150_12220-26647--,00.html