A prion is a type of protein that can cause disease in animals and humans by triggering normally healthy proteins in the brain to fold abnormally. he prion mode of action is very different to bacteria and viruses as they are simply proteins, devoid of any genetic material.

Once a misfolded prion enters a healthy person – potentially by eating infected food – it converts correctly-folded proteins into the disease-associated form. To date, nobody knows quite how this happens.

Prions in "mad cow" brain. Coloured transmission electron micrograph (TEM) of prion fibrils in the brain of a cow infected with BSE (Bovine Spongiform Encephalopathy) or "mad cow" disease. Prions are virus-like organisms made up of a prion protein.

These elongated fibrils (green) are believed to be aggregations of the protein that makes up the infectious prion. Prions attack nerve cells producing neurodegenerative brain disease. "Mad cow" symptoms include glazed eyes and uncontrollable body tremor. Prions cause BSE in cattle; scrapie in sheep and goats; and Creutzfeldt-Jakob disease in humans.