CDC/Teresa Hammett; Sherif Zaki MD PhD; Wun-Ju Shieh MD PhD MPH [Public domain] via Wikimedia CommonsPrions are abnormal proteins whose accumulation in brain tissues can cause disease in their hosts. Prion diseases occur in many different species, and some can be transmitted between species. For example, variant Creutzfeld-Jakob disease occurs in humans after exposure to the cow prion disease bovine spongiform encephalopathy (or mad cow disease). Generally, such inter-species transmission is limited by biological differences between hosts. But new research suggests that this barrier may be weaker than previously thought, in part because prions can be transmitted through tissues other than the brain.
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