Jill Lepore discusses the “stay at home” campaigns that ran on radio stations during the polio years, devised to keep children indoors; she is especially fond of a program that featured a young Hubert Humphrey reading comics. Lepore finds solace in revisiting the desperate measures of that era. “One of the reasons I study history,” she says, “is I like to see how things began, so I can imagine how bad things end.” She describes the momentous day, in 1955, when Dr. Jonas Salk and his colleagues announced the success of the polio vaccine trials. “That’s the great blessing of a vaccination program,” Lepore says. “We forget how bad the disease was.” Plus, David Remnick speaks with three mayors who have to negotiate the task of reopening their cities safely.