THE SPREAD of Ebola has added a scary twist to one of the clichés of our age: that we live in a world of shrinking distances. Boston isn’t one of the five US airports where officials will aim an infrared thermometer gun at anyone coming off a plane from West Africa. But passengers who reached Logan Airport with flu-like symptoms last week were escorted to hospitals by a team in hazmat suits, and our eyes now scan the horizon nervously, wondering about every new arrival.

New as it might seem, this anxiety about our hyperconnectivity has a long lineage: In the 17th and 18th centuries, Bostonians felt a similar terror. The ships that streamed into Boston Harbor from around the Atlantic world carried a vital lifeblood—the commerce that built Boston—but they also carried the microbes of infectious disease.

The most fearsome of all was smallpox, the disease that wiped out so many Native Americans at the time of European settlement, and that also killed large numbers of the English. A terrible epidemic came in 1721, infecting roughly half of Boston’s 11,000 residents. But Boston’s approach to public health changed that year, thanks to an experimental strategy for inoculating citizens with small traces of the disease.

The idea behind this radical new treatment came from Africa, specifically from a slave named Onesimus, who shared his knowledge with Cotton Mather, the town’s leading minister and his legal owner. Boston still suffered dreadfully, but thanks to Onesimus and Mather, the terror linked to smallpox began to recede after Africans rolled up their sleeves—literally—to show Boston how inoculation worked. The story of how Boston began to overcome smallpox illustrates the strife that epidemics can cause, but also the encouraging notion that humans can communicate remedies as quickly as they communicate germs—and that the solutions we most need often come from the places we least expect to find them....