The prion is a protein, thought to linger in human tissue, potentially for many years, before any symptoms begin to emerge.

BBC: vCJD and BSE - the link

But it tested positive for the causal agent for BSE, a protein called a prion, which can arise spontaneously in elderly cattle.

MSN: Saudi Arabia bans Brazil beef imports on mad cow doubts

The odd thing about prion diseases is that the misfolded protein in question somehow acts as a catalyst for other protein molecules of the same type to misfold in the same way.

ECONOMIST: Creutzfeldt-Jakob disease