Scientists find deadliest pandemics were caused by same plague The plague is usually spread to humans by rodents whose fleas carry the bacteria. "Humans are infringing on rodents' territory, so it's only a matter of time before we get more exposure to them," Mr Gilbert said. Still, he and other experts doubted a modern plague epidemic would be as devastating. "Plague is something that will continue to happen but modern-day antibiotics should be able to stop it," said Hendrik Poinar, director of the Ancient DNA Centre at McMaster University in Canada, who led the new research. He said about 200 rodent species carry the plague and could potentially infect other animals or humans. Mr Poinar warned that if the plague transforms into an airborne version - which can happen if the bacteria reaches the lungs and its droplets are spread by coughing - it would be much harder to snuff out. That type of plague can kill people within 24 hours of being infected. He said scientists need to sharpen their surveillance of plague in rodent populations to try averting future human infections. "If we happen to see a massive die-off of rodents somewhere with (the plague), then it would become alarming," he said. There are several thousand human cases of plague every year, most often in central and Eastern Europe, Africa, Asia and parts of the Americas.