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Medieval diseases are making a grim comeback

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.

Plague is caused by the bacterium Yersinia pestis, which is usually carried by rat fleas and passed on to humans through biting. The resulting infection adheres to a gruesome timetable. It takes between one and seven days for a fever and nausea to develop; then come the dreaded “buboes”, or swellings in the groin and armpits, which give bubonic plague its name. If the bacteria invade the bloodstream, necrosis sets in; the limbs darken as they rot. Left untreated, the condition carries a 30-60 per cent chance of death.

Should the infected pus reach the lungs, bubonic plague can become pneumonic plague. This rare form, even deadlier than its bubonic cousin, is highly infectious because it can be transmitted through airborne droplets.

The epidemic in Madagascar is, alas, mostly pneumonic: it totals nearly 700 cases, 57 deaths and 11 distinct strains of Yersinia pestis. It garnered special attention because it arrived in August, earlier than expected (the endemic plague season usually runs from September to April) and because cases appeared in historically unaffected regions. The WHO has released funding for antibiotics and surveillance.

The disease has spread beyond the island: a visitor has been identified, in his home country of the Seychelles, as a probable case of pneumonic plague. His contacts have received antibiotics. Meanwhile, Air Seychelles has suspended flights to and from Madagascar.

The Madagascan outbreak follows a puzzling 2016 episode that unfolded in a remote, unstable part of the country. Reaching the region, Befotaka in southeastern Madagascar, requires a two-day drive and three days of walking. In 2016, health officials and scientists twice visited Befotaka to take samples, treat patients and carry out pest control (the black rats known to spread the plague can sometimes be found living in houses). Both visits had to be cut short because the teams received threats. A third visit this year revealed 126 suspected cases of plague.

It is no coincidence the disease has found fertile pickings in a troubled part of the country characterised by the lack of clean water, electricity, nutrition and basic health provision. History shows that natural disaster, poverty and war, along with the absence or destruction of protective infrastructure, threaten our ability to keep pathogens at bay.

The world bears this out today: war-torn Yemen is in the grip of the worst cholera epidemic of modern times. More than 700,000 Yemeni, mostly children, are affected. Many of them are malnourished, complicating treatment.

Meanwhile, drug-resistant forms of tuberculosis have emerged worldwide. Unvaccinated children, particularly in conflict zones and impoverished regions, are succumbing to conquerable diseases such as polio and rubella. The haunting truth is that future generations are still being struck down by diseases of the past.

The writer is a science commentator