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 today in the journal Science, are strong evidence for the "protein-only hypothesis," the controversial idea that a protein, acting alone without the help of DNA or RNA, a cousin of DNA, 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, San Francisco,...