Chronic wasting disease (CWD), sometimes called zombie deer disease, is a transmissible spongiform encephalopathy (TSE) affecting deer. TSEs are a family of diseases thought to be caused by misfolded proteins called prions and include similar diseases such as BSE in cattle, Creutzfeldt–Jakob disease (CJD) in humans and scrapie in sheep. Natural infection causing CWD affects members of the deer family. In the United States, CWD affects mule deer, white-tailed deer, red deer, sika deer, elk, caribou, and moose. The transmission of CWD to other species such as squirrel monkeys and humanized mice has been observed in experimental settings.

The mule deer is a deer indigenous to western North America; it is named for its ears, which are large like those of the mule. Two subspecies of mule deer are grouped into the black-tailed deer.