Scientists are reporting that, for the first time, they have made an artificial prion, or misfolded protein, that can, by itself, produce a deadly infectious disease in mice and may help explain the roots of mad cow disease. The findings, being reported on Friday in the journal Science, are strong evidence for the so-called "protein only hypothesis," the controversial idea that a protein, acting alone without the help of DNA or RNA, can cause certain kinds of infectious diseases. The concept was introduced in 1982 by Dr. Stanley Prusiner, a neurology professor at the University of California in San Francisco,...