Prions are the most bizarre infectious agents ever imagined. It was Stanley Prusiner of the University of California, San Francisco, who first discovered the nature of prions and suggested they are the causative agents of the spongiform encephalopathies. For this he won the prestigious Lasker Prize two years ago.

Prions (pronounced pree-ons ) are proteins, rogue proteins, and nothing else. They contain no nucleic acid (DNA or RNA). They consist of a single molecule containing about 250 amino acids, termed the PrP protein. They are abnormal variants of proteins that occur normally in cells, such as human brain cells. It seems that a protein alone is the infectious agent.

A prion has been defined as "small proteinaceous infectious particles which resist inactivation by procedures that modify nucleic acids". The discovery that proteins alone can transmit an infectious disease has come as a considerable surprise to the scientific community.Prion diseases are often called spongiform encephalopathies because of the post mortem appearance of the brain with large vacuoles in the cortex and cerebellum, as can be seen in the figure below. Probably most mammalian species develop these diseases. Furthermore, there are many similarities between prions from different mammals.

Prions are the only "life forms" that break the great "central dogma" of biology. That is, we have come to expect that all life forms from viruses to bacteria to plants to humans to hand down the blueprints for all their progeny via their DNA (except for some viruses which carry their blueprints as an RNA genome), and we expect that the process for converting the blueprints into building blocks must involve replication of DNA, transcription of the message into RNA, and translation of the RNA s message to form proteins, the building blocks of cells, tissues, organs and whole organisms. Here we have life forms where abnormal proteins, the PrP proteins, direct the refolding of normal proteins just by direct contact.