Scrapie is a disease that attacks the brains of sheep, producing strange behaviors such as the animal's compulsion to scrape off its wool, which gives the disease its name. Scrapie has been known to scientists for more than 250 years, although virtually nothing about its nature or transmission was discovered until very recently. In 1986, a scrapie-like disorder was first noted in a herd of cows in Kent, England. Within five years, the disease had spread to cows throughout the country. Cows inflicted with the disease became very sensitive to sound and touch, walked unsteadily, and trembled and twitched. Scientists were able to determine that scrapie--one in a family of diseases termed spongiform encephalopathies--had been transmitted from sheep to cows. Studies of brains from infected animals showed some common features, especially the presence of rod-shaped particles that clumped together to form larger arrays.

What was the infectious agent in this...