“Prions,” Harding said, “are the simplest disease-causing entities known, even simpler than viruses. They’re just protein fragments. They’re so simple, they can’t even invade a body—they have to be passively ingested. But once eaten, they cause disease: scrapie, in sheep; mad-cow disease; and kuru, a brain disease in human beings. And the dinosaurs developed a prion-disease called DX, from a bad batch of sheep protein extract. The lab battled it for years, trying to get rid of it.”
