The Middle Ages were haunted by one ghost in particular: the bubonic plague. But the disease, which wiped out one third of the European population in earlier times, kills people in many areas of the world even today. Recently, antibiotic-resistant strains of the plague bacterium, Yersinia pestis (right), have cropped up, making it even more important to understand epidemics of the disease. In an entirely theoretical paper published in Nature today, M. J. Keeling and C. A. Gilligan of the University of Cambridge in England point out the importance rats play in keeping the disease going and offer advice about how to contain it.

Bubonic plague is primarily a disease of rodents, spread by fleas. The fleas feed on an infected rat and soon become infectious. When the rat dies, they typically find a new host and thus distribute the disease in a rat population. Only when the number of rats becomes very low are the fleas forced to seek a different host. This is how they jump to human hosts, leading to an epidemic. Despite this life cycle, most plague models up to now have concentrated only on human populations.

The researchers have now calculated the proportion of susceptible rats that determines whether the disease spreads to humans. If few rats are susceptible, the plague is likely to remain endemic. Only when more than 80 percent of the rats are susceptible do humans succumb. The plague bacterium can survive for many years in only a small number of rat subpopulations, which explains why the plague crept up again and again in European cities in the 15th and 16th century, despite efforts to contain it.

Vaccination of humans is ineffective in eradicating the disease, because it persists in the rat population. According to the scientists, keeping the number of rats low is a good alternative, but the right timing is crucial: If the rat density is kept consistently low, the disease has less of a chance to break out in rats. If action is taken only after the first human cases have appeared, though, exterminating rats can have the opposite effect: infected fleas, deprived of their hosts, will jump to humans more frequently.