This is the VOA Special English Health Report.

In December, the United States reported its first case of mad cowdisease. Agriculture officials have been working to make sure othercows that came from Canada with that animal are not used as food.This is because of the possibility that people who eat infected beefcan get a human form of the disease. Officials say more thanone-hundred-forty people in Britain have died since an outbreak ofmad cow disease in the nineteen-eighties. Ten deaths have beenreported in other countries.

The scientific name of the cattle disease is bovine spongiformencephalopathy. A similar disease in sheep is known as scrapie. Deerand elk suffer chronic wasting disease. And minks get a diseasecalled transmissable mink encephalopathy.

In humans, there is a disease in babies called Alpers syndrome.Other similar diseases in people include fatal familial insomnia,kuru and Creutzfeld-Jacob disease. The human version of mad cowdisease is a form, or variant, of Creutzfeld-Jacob.

All these diseases create holes inthe brain. All kill. And experts say all are caused by infectiousproteins. American scientist Stanley Prusiner discovered this kindof protein in the early nineteen-eighties. He named it a "prion"[PREE-on]. He won the Nobel Prize for Medicine. Prions contain nogenetic material. So they cannot copy themselves the way bacteria,viruses and other infectious agents do.

Prions are found naturally in brain cells of people and animals.Research published last month in Cell magazine suggested that prionscould help the brain store memories. But experts are not sure oftheir purpose.

Prions appear to do no harm until one changes shape. Normally aprion is round like a ball. The protein becomes dangerous when itunfolds into a straight line. When it touches another prion, thesecond one unfolds and touches another. That one also unfolds, andso on.

This process can start naturally. Or it can begin when anunfolded prion enters the brain of a person or animal that has eateninfected tissue. This is why farmers are now banned from feedingcows the remains of other cows and sheep.

Researchers are trying to discover more about prions. A bigquestion is why only some people who ate infected beef have gottensick.

This VOA Special English Health Report was written by NancySteinbach.