The Great Plague (AD 1665-1666) was a massive outbreak of disease in England that killed 75,000 to 100,000 people, up to a fifth of London's population. The disease is generally believed to have been bubonic plague, an infection by the bacterium Yersinia pestis, transmitted via a rat vector. Other symptom patterns of the bubonic plague, such as septicemic plague and pneumonic plague were also present.