How can people catch mad cow disease from eating cooked beef? Why doesn't cooking kill the germs?
Mad cow disease, otherwise known as bovine spongiform encephalopathy (BSE), is thought to be caused not by a bacteria or virus, but by a type of protein called 'prions' from 'proteinaceous infectious particles'.
The prions are highly resistant to things that normally damage proteins such as heating to very high temperatures, acid, ultraviolet radiation and organic solvents.
When transmitted through eating, the prions are believed to be absorbed in areas called Peyers patches in the digestive tract. From there, the prions move to nerve and brain cells where they are thought to cause degenerative diseases such as Creutzfeld-Jakob disease in humans.
The idea that a simple protein could cause disease was first put forward by Professor Stanley Prusiner of the University of California's School of Medicine in 1982.
At the time the idea was considered controversial. The medical community believed that diseases required genetic material, composed of nucleic acid (DNA or RNA), in order to establish illness in an animal or human host.