Fast-food joints, long inhospitable to any kind of labor activism, are suddenly beset by a surge in strikes. Over the past few months, workers at chains such as McDonalds Corp. have walked off the job in more than 60 cities, demanding a “living wage” of $15 an hour. Regardless of whether the strikes lead to better pay, they have rekindled debate over what constitutes a living wage.

That debate, however, has stranger, older and more curious origins than either proponents or detractors of the living wage might imagine.

The story begins in medieval England in the 14th century. Life, never particularly easy at this time in history, had become especially nasty, brutish and short. The preceding year, the “Great Pestilence,” better known as the Black Death, had arrived in continental Europe. The pandemic, one contemporary noted, “began in India and, raging through the whole of infidel Syria and Egypt,” reached England in 1349, “where the same mortality destroyed more than a third of the men, women and children.”...