The infective agent is called a prion and is a normal brain protein that has changed shape and cannot be broken down in the brain (and is also very heat resistant). When this prion comes into contact with normal brain protein it causes it to also change its shape and the accumulation of this abnormally shaped protein gradually destroys brain tissue. Since the 1920s some animal feed has contained the rendered remains of other animals including cattle. This feed (now totally banned) was certainly the cause of the epidemic because it re-cycled prions from other cattle that were incubating BSE. It is uncertain how this feed initially became infected because BSE was unknown prior to 1985 but perhaps a single mutant cow producing lots of BSE prion was the initial source of infection in feed.