BUBONIC plague brought terror to medieval Europe. Over a third of its population perished from the “Black Death” in the 14th century, hastening the end of the feudal system. As a bacterial disease, the plague these days is generally treatable with modern antibiotics. Nonetheless, it persists beyond the grim chapters of history. On June 26th health authorities in New Mexico, in the south-western United States, announced that three people had been diagnosed with the disease in the previous month alone. This is a marked uptick for a country that records around seven cases a year nationwide, according to the US Centres for Disease Control and Prevention.

Zoonotic diseases such as the plague, Ebola and avian flu—which are generally carried by animals—are extremely hard to eradicate. The plague is caused by a bacterium, Yersinia pestis, which infects fleas, which in turn live mainly on rodents. In Europe, those fleas lived mostly on black rats. In America’s south-west, the site of most cases observed in the rich world, the fleas have shifted to rural squirrels and prairie dogs.

No vaccine has been developed for the plague, and if the illness is not treated quickly with drugs the death rate is high. The most common form is the bubonic plague, which is spread by flea bites or by contact with animals, and which kills 30-60% of those infected. A rarer pneumonic form, which spreads to the lungs and can be transmitted by sneezing or coughing, is invariably fatal without treatment. In America four people died of the plague in 2015, its highest annual toll for 30 years.

Worldwide, however, the plague is mainly a disease of poverty. Natural focuses—areas in which the bacteria, fleas and animal reservoirs might create the right conditions for the plague to spread—are found in much of the world. But most cases occur in countries where people live in unsanitary conditions, and where treatment may be slower.

Between 2010 and 2015 there were 3,248 cases and 584 deaths. The worst-affected country is Madagascar: three-quarters of all new infections and deaths occur there. And the disease is springing up in new places on the island. In January the World Health Organisation confirmed that 62 cases were reported in districts of Madagascar that had not seen an outbreak since 1950, killing 26 people. The ancient killer may be less deadly than in the past, but it has not gone away.