Like mad cow disease in cattle, scrapie in sheep, and Creutzfeldt-Jakob disease in people, CWD is an incurable and fatal condition that afflicts whitetails, mule deer, elk, and moose. All are "transmissible spongiform encephalopathies," which researchers believe are caused by mysterious, nonliving proteins known as prions. Prions accumulate in lymphatic and nerve tissues, riddling a victim's brain with holes (the "sponge" in spongiform), and in the process causing a horrific death. CWD was first discovered in the 1960s in captive cervids in Colorado. In 1981, the first wild cases appeared in Rocky Mountain National Park. Over the next few decades, it spread through free-ranging deer and elk populations in Colorado and Wyoming, and eventually into adjacent states.