Prion disease epidemics, which have been unpredictable recurrences, are of 
significant concern for animal and human health. Examples include kuru, once the 
leading cause of death among the Fore people in Papua New Guinea and caused by 
mortuary feasting; bovine spongiform encephalopathy (BSE) and its subsequent 
transmission to humans in the form of variant Creutzfeldt-Jakob disease (vCJD), 
and repeated examples of large-scale prion disease epidemics in animals caused 
by contaminated vaccines. The etiology of chronic wasting disease (CWD), a 
relatively new and burgeoning prion epidemic in deer, elk, and moose (members of 
the cervid family), is more enigmatic. The disease was first described in 
captive and later in wild mule deer and subsequently in free-ranging as well as 
captive Rocky Mountain elk, white-tailed deer, and most recently moose. It is 
therefore the only recognized prion disorder of both wild and captive animals. 
In addition to its expanding range of hosts, CWD continues to spread to new 
geographical areas, including recent cases in Norway. The unparalleled 
efficiency of the contagious transmission of the disease combined with high 
densities of deer in certain areas of North America complicates strategies for 
controlling CWD and raises concerns about its potential spread to new species. 
Because there is a high prevalence of CWD in deer and elk, which are commonly 
hunted and consumed by humans, the possibility of zoonotic transmission is 
particularly concerning. Here, we review the current status of naturally 
occurring CWD and describe advances in our understanding of its molecular 
pathogenesis, as shown by studies of CWD prions in novel in vivo and in vitro 
systems.