Many epidemics of fever ravaged Europe from ancient times through the early 20th century. But one disease stands out in historical accounts because authors describe patients appearing to recover before relapsing into fever again and again. This disease has been around for so long that Hippocrates described a series of such fevers that struck the city of Thasos in the wake of an especially harsh winter, and outbreaks have persisted through last century.

The disease tended to show up when times were hardest. Over the centuries, records describe epidemics of a nearly identical illness, usually on the heels of war or famine, with isolated cases popping up between times among the poor. One such epidemic struck during the Great Irish Famine of 1846 to 1852. Another ravaged Central Europe and Russia in the aftermath of World War I, killing at least five million people.

For years, historians have blamed those epidemics, termed louse-borne relapsing fever (LBRF), on Borrelia recurrentis, a twisting, spiral-shaped bacterium transmitted only by the human body louse. The logic was simple: B. recurrentis causes the only relapsing fever we know of that’s carried by lice and capable of spreading fast enough to cause an epidemic. Although it seems to make frequent and horrible appearances in the historical record, LBRF has been totally invisible in the archaeological record. A new study changes that and provides evidence that B. recurrentis is indeed at fault.