Prion protein plaque (right) surrounded by nerve cells (seen mainly on the left), computer artwork. Prions are infectious agents that cause a group of fatal neurodegenerative diseases, transmissible spongiform encephalopathies (TSEs), by aggregating within the central nervous system. The diseases caused include scrapie in sheep, BSE in cows and CJD in humans. Unlike other infectious agents, prions do not have a nucleic acid genome for replication. Instead, the abnormal infectious prions directly change normal prions found in cell membranes to the abnormal form. This results in resistance to digestion by enzymes, a build up of protein plaques and death of the surrounding brain and nerve cells.