Prion diseases have been recognized in several animal species and include scrapie in sheep, bovine spongiform encephalopathy (BSE or “mad cow disease”) in cattle, chronic wasting disease (CWD) in deer and elk, and transmissible mink encephalopathy (TME) in mink. In humans, the most common form of prion disease is Creutzfeldt-Jakob disease (CJD). A new form of CJD in humans called variant CJD (vCJD) has been linked with exposure to BSE-contaminated materials. Thus, animal prion diseases have become a significant public health issue and have raised concerns about food safety. In light of the fact that BSE has caused a new form of prion disease in humans, as well as continuing uncertainty as to whether or not other animal prions may do the same, it is critical to understand how animal prions infect humans.