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Like no other disease, plague evokes terror. One of the most lethal illnesses in human history, it killed probably a third of Europe's population in the 14th century. It may also have been one of the first agents of biological warfare: It's said that in the 1340s, invading Mongols catapulted their plague dead over the city wall into Kaffa in the Crimea.

Yet the plague is not just a disease of the distant past. While cases tapered off in the mid-20th century, the World Health Organization (WHO) now classifies plague as "re-emerging." No one is predicting another pandemic like the Black Death that devastated Europe. The WHO now records at most only a few thousand cases worldwide per year; and, if detected early, the disease can be treated effectively with antibiotics. But since the early 1990s, plague has returned to places — including India, Zambia, Mozambique, Algeria and parts of China — that had not seen it in many years or even decades. Its global footprint has also shifted, according to a paper published last month in the journal PLoS Medicine. In the 1970s, most plague cases were in Asia; today, more than 90% are in Africa. The conundrum for epidemiologists: Why is human plague reappearing now, even though nearby animal populations have likely harbored the culprit Yersinia pestis bacteria all along?

Plague lives in many rodent species, and is most often transferred to humans by the animals' fleas. Scientists know which regions of the world harbor infected animals, but they are only just beginning to understand the dynamics of plague infection. Its spread depends not just on Yersinia pestis but also on interactions among rodents and, crucially, on contact between humans and wildlife. Madagascar is a good example. For decades, plague was restricted to the highlands, according to a 2004 paper by researchers in Madagascar, Senegal and France. But it showed up on the coast in 1991, when the Asian shrew somehow picked up infected fleas. The plague's earlier comeback in the inland capital, Antananarivo, arose as city sprawl and shoddy housing put residents in closer contact with black rats. In 1998, inland villages reported cases, too, perhaps caused by rats displaced through deforestation.

Even in the antibiotic age, then, containing plague requires monitoring more than human cases, says Nils Christian Stenseth, head of the Center for Ecological and Evolutionary Synthesis in Oslo, and lead author of the PLoS Medicine paper. Working with nearly 50 years of animal, human and bacteriological statistics from the former Soviet Union, his team found that human plague in Kazakhstan occurs only when the local gerbil population reaches a certain threshold in winter. Warmer winters mean more gerbils. That, says Stenseth, suggests plague's "re-emergence might have a climate component."

If so, global warming may exacerbate the threat — an unsettling thought, given the viciousness of the disease. "The plague bacillus is probably the most pathogenic infectious agent on the planet right now, and we still don't know why it's so virulent," says Elisabeth Carniel, a plague expert at the Institut Pasteur in Paris. It may no longer make history, but plague hasn't lost its terrifying power.

• Laura Blue