Mice infected with hamster prions remain apparently healthy throughout their normal life span, but in fact become highly infectious. Cattle are believed to be infectious at an early stage of incubation as the disease spreads through the central nervous system toward the brain, the most lethal tissue of all. The possibility that an animal can be infectious and show no symptoms raises the question whether people can as well. Scientists fear, for example, that a patient with undetected Creutzfeldt-Jakob disease who was undergoing surgery treatment for another disease might pass it along through surgical instruments. Since nobody knows the average incubation period in man, blood transfusion services in several countries, including the United States and Canada, are turning away donors who have lived in Britain although it is not certain that the defective prions can be passed on through blood.
Proteins are the body's primary component and the basis of all enzyme reactions.
As they are produced, they fold or coil three-dimensionally. The agent that causes spongiform disease is a protein that has folded wrongly, and which is able to pass this defect to normal proteins. Because the defective prions resist breakdown by enzymes, they build up within nerve cells and eventually the brain. It is as though bricks told an architect how to build a house. Unlike viruses, proteins contain no genetic material and therefore provoke no immune response.