“Mystery of the Black Death” explores why the plague that swept through Europe periodically from the 14th through 17th centuries didn't kill everyone who was exposed to it. The hour follows American geneticist Stephen O'Brien as he visits Eyam, a small town in the north of England, to try to determine why many in the town survived an outbreak in 1665-66 or didn't get sick at all. The answer: “a tiny genetic mistake” known as Delta 32, says narrator Liev Schreiber.