After bubonic plague reached Europe in the 14th century, major cities such as Venice enacted strict quarantines intended to keep out infected, shipborne rats. The cities would remain free of the disease for a time, but every decade or so, the plague returned. People have presumed that infected rats had sneaked back into town.
Challenging that assumption, computer modeling of how bubonic plague spreads from rats to people now suggests that the disease never went away. It probably persisted in the rat populations living in the cities, say Matthew J. Keeling and Chris A. Gilligan, both of Cambridge University in England.
Beyond suggesting that bubonic plague can lurk within rat populations for years without causing human epidemics, Keeling and Gilligan's work published in the Oct. 19 Nature comes to the counterintuitive conclusion that killing rats may sometimes exacerbate plague outbreaks among people.