A degenerative disease of the brain that affects cattle and is caused by an abnormal form of a cellular protein (see prion). Known colloquially as ‘mad cow disease’, it results in a build-up of amyloid tissue in the brain. The infective agent can be transmitted to other cattle via feed containing offal derived from infected animals. It can also, under certain circumstances, be transmitted to other species. Humans infected by eating contaminated beef or beef products have developed a variant form of Creutzfeldt-Jakob disease.