Ninety-two people, most in Britain, are believed to have contracted a human form of the disease from eating the meat of infected cattle. Unlike viruses or bacteria, prions are not life forms in themselves, but merely protein molecules that can become misshapen. Brains are full of normal prions, but if abnormal prions appear in the brain, they can cause disease by converting normal protein molecules nearby into their deadly form. Deposits, called plaques, start to form in the brain. Eventually the brain becomes full of holes, like a sponge, and can no longer control vital functions.
Mad cow is one of a family of prion diseases that strike humans and other animals.
CJD was around long before mad cow disease appeared, and is not linked to beef consumption. About one in a million people every year get CJD, which appears to be caused by a different abnormal prion. It can crop up spontaneously, be related to genetic disorders, and in rare cases be transmitted by brain surgery or corneal transplants. People often become confused by the terminology since the human form of mad cow disease found in Europe is called new variant Creutzfeldt-Jakob disease. Nearly all cases of infection in cattle have occurred in Britain, though in the last several months, infected animals have turned up in France, Germany, Portugal, Spain, Ireland, Switzerland, Belgium, Denmark, Italy, the Netherlands, Luxembourg and Liechtenstein.