Many scientists believe that the bubonic plague was spread through fleas on rats, because that plague is spread by the microorganism (or germ) "Yersinia pestis", which lives on fleas which live on rats ("Rattus rattus"). Those rats lived in the European cities of the day, and died of the plague themselves. Some scientists believe that the plague spread faster than the rats. If this is true, the rats cannot be the main carrier. More research is needed to find out if this is true. People believe this disease was the 'Black Death'. It killed nearly a third of the population of Europe, in many epidemics in the Middle Ages.