Just what causes mad cow disease is not known. The prime suspect is a prion, an aberrant protein that attacks the central nervous system. Scientists are used to bacteria and viruses, but prions are a new kind of infectious agent. The theory is that herds in Britain were infected from contaminated bone meal. BSE crossed from animals to people, causing a variant of the rare degenerative brain disorder called Creutzfeldt-Jakob disease.
Greatest concern is focused on brain and bones and mixed meats such as sausage.
The disease is horrible and invariably fatal. About 300 Americans die of the classic form of Creutzfeldt-Jakob disease a year, which strikes seemingly at random. There is no effective treatment. With listeriosis, for example, an estimated 2,500 Americans get very sick, but they can be treated with antibiotics and most of them recover. Mad cow disease is a death sentence, and victims are generally younger than those who succumb to the classic form of the brain malady.