Have you ever wondered what caused mad cow disease? It is not caused by bacteria, viruses or parasites. In fact, mad cow disease is instead caused by infectious molecules called prions, which are not even technically alive and don’t have DNA or RNA.

Prions are a type of protein. You can think of a protein as a chain of little molecules called amino acids. This chain will fold up into a specific shape that is essential for the protein’s job in the cell. There are normal prions that are correctly folded and are not infectious. Infectious prions, like those that cause mad cow disease, are misfolded versions of the prion protein.

So how do they work? Infectious prions spread the misfolding to their normal counterpart proteins. To describe how this works, imagine a single infectious prion. When it comes in contact with a normal prion protein, this will result in the normal protein being misfolded too. If those misfolded prions each find another normal prion protein, this process will continue until the misfolded prions build up and start killing the brain tissue.

Prions can multiply in the brain for 10+ years before any symptoms become apparent. At this point however, the disease is fatal within months.

The reason why so many people were concerned about mad cow disease a few years ago was because it was possible that humans who consumed infected beef could contract a prion disease since prions are almost impossible to destroy; cooking infected meat at a normal cooking temperatures is not enough to break down the proteins.

Most prion diseases in humans (~85%) occur sporadically – i.e. we don’t know why they happen. One theory is that one prion protein in the normal cell misfolds by chance, and the disease spreads from there. Some prion diseases (~15%) are also genetic.

Research into prion biology and work on a cure continues to this day.