Twenty-five year ago, little was known about the causes of neurodegenerative diseases.  Now, however, it is clear that they result from abnormalities in the processing of proteins. Of all the laboratory research done on neurodegenerative diseases, the studies that led to the discovery of prions, infectious proteins, have yielded the most unexpected findings. The concept of prions has provided an explanation of how a disease can be both infectious and genetic, and has revealed previously unknown kinds of neurologic disease. (#7201)