The Black Death was little short of a bacterial apocalypse. The outbreak of bubonic plague, imported along the Silk Road, is thought to have killed between 25m and 50m people as it rampaged through 14th-century Europe. The disease thence resurfaced sporadically: the Great Plague of London, for example, felled a fifth of city dwellers in the 1660s.
While the plague seems to us a medieval affliction, it has never fully disappeared. On average, about 500 cases are documented globally each year, mostly in Africa, South America and India. The infection is treatable with antibiotics if caught early.
Now the World Health Organization has noted an unusually large outbreak of plague in Madagascar. One case has also been reported in the Seychelles. The threat is very likely to be contained but the resurgence of this historic pestilence demonstrates the fragile biological stand-off between human and bacterium.