Francesco I de’ Medici, the founder of the Uffizi Gallery, and his mistress-turned-second wife, Bianca Cappello, died agonizing deaths within one day of each other in 1587. For centuries, the suspicious believed that Francesco’s brother and successor, Ferdinando, had them poisoned, likely with arsenic. But recent tests have shown that Ferdinando was innocent (well, in this instance). What killed Francesco and Bianca? Just what the original examiners thought killed them, at it turns out.