On August 3, 1900, a woman who sold fish along the River Clyde in Glasgow, Scotland, started vomiting and couldn't stop. Within days, she and a granddaughter who shared her tenement were dead. Soon, health officials would confirm their worst fears: “Mrs. B” had contracted Bubonic plague—the first outbreak in the United Kingdom in centuries.

Authorities moved quickly. They reached out to anyone who’d had contact with the deceased. They tracked down a sick child most likely infected by a babysitter who’d attended the grandmother’s wake. They traced a factory worker’s illness to colleagues who’d visited the victims’ family. Health officials isolated at least a hundred people. They disinfected sheets and fumigated apartments. By the time they contained