Sometimes taking more than 30 years to display symptoms, the diseases slowly attack brain tissue, often leaving spongelike holes. They are characterized by accumulations of abnormal forms of a protein, called prion protein, which, unlike viruses or bacteria, contain no genetic material and have no known ability to reproduce themselves. Normal prion proteins occur naturally in brain tissue. The abnormal form differs in shape from the normal prions and is not susceptible to enzymes that normally break down proteins. In the brain, abnormal prions appear to increase their number by directly converting normal prions.

Prion diseases have both infectious and hereditary components. The gene that codes for prions can mutate and be passed on to the next generation. Most of the diseases also can be acquired directly by infection, but unlike other infectious agents, prions provoke no immune response. Most prion diseases, however, are not highly transmissable; chronic wasting disease is the exception because infected deer that have not developed the disease shed prions from lymph tissue in their intestines, contaminating the soil and plants on which other deer graze with the prions in their feces.

An epidemic of BSE in Great Britain that was diagnosed in 1986 and infected some 178,000 cows appears to have been caused by a protein feed supplement that contained rendered remains of scrapie-infected sheep brains. In 1996 a suspicion that BSE had been transmitted to humans who died of a variant of CJD in Britain caused a scientific and economic furor as the European Union imposed a ban (1996) on the export of British beef, which was partially lifted in 1999 and fully lifted in 2006. The U.S. Dept. of Agriculture banned the import of cattle and many cattle byproducts from most European nations because of BSE. Instances of BSE in cattle have also occurred in many other European countries, Canada, the United States, and Japan, but the vast majority of cases occurred in Britain in the 1980s. There is now compelling evidence that BSE is the same disease as variant CJD (vCJD), which has killed less than 200 people, but it is not yet known exactly how the disease is passed from animals to humans.

The idea of disease-causing protein particles was first put forward in 1981 by Stanley B. Prusiner, the neurologist who coined the term prion (from proteinaceous infectious particle). The prion theory has been controversial from the beginning, and although scientific evidence for the existence of such infectious particles has increased, an exact causal link between prions and the diseases they are believed to cause remains to be established. Critics believe that these diseases are caused by unidentified viruses.