Now, in some very rare cases these misfolded proteins can actually form these biological molecules we call prions. And prions are very, very dangerous molecules as we'll see in just a moment. Prions are these infectious agents that can actually cause a variety of different types of diseases not only in humans but also in other animals. For example, mad cow disease in cattle and screeping sheep is caused by these prion molecules. In fact, in humans a disease known as crytesval yaka disease which is a deadly disease. It's called by prion molecules.