Abstract: Ongoing endemics and epidemics of prion disease afflict several species of ruminants regularly consumed by humans. Bovine spongiform encephalopathy (BSE) is epidemic in British cattle, and is now found in the cattle of more than 20 countries. A large, and apparently growing, epidemic of chronic wasting disease plagues deer and elk in North America. Finally, scrapie has been endemic in the sheep of most countries for many decades. It was once assumed that humans were not susceptible to these ruminant forms of prion disease, but an outbreak of a new form of Creutzfeldt-Jakob disease (CJD) among young Britons, almost certainly due to dietary exposure to BSE-contaminated beef, has disproved this supposition. Although all prion diseases share the same fundamental pathologic mechanism, transmission between species is sometimes inefficient. The basis of this “species barrier” is incompletely understood, but interspecies differences in the amino acid sequence of the prion protein and the strain of prions involved play critical roles. Reliable experimental models for determining the resistance of humans to animal prion diseases do not yet exist. It is possible that animal to human transmission of prion disease may manifest as CJD with unusual characteristics, but this is not necessarily the case. In the absence of a reliable means for determining the susceptibility of humans to animal prion disease, measures to minimize human exposure to animal prions should be emphasized.