Mad cow disease, composite image. At top left is a molecular model showing the secondary structure of a prion protein, the causative agent of bovine spongiform encephalopathy (BSE) in cows and Creutzfeldt-Jakob disease (CJD) in humans. At bottom right are prions and a computer model of the brain. Above this are brain scans showing aggregations (pink) of prions. Unlike other infectious agents prions do not have a nucleic acid genome for replication. Instead, the abnormal infectious prions change normal prions, found in cell membranes, to the abnormal form. This leads to a build up of protein plaques. BSE and CJD are fatal neurodegenerative diseases characterised by progressive dementia and a rapid deterioration in co-ordination.