 The Great Plague killed an estimated 100,000 people—almost a quarter of London's population—in 18 
months.The plague was caused by the "Yersinia pestis" bacterium, which is usually transmitted through the bite of an infected rat flea.The 1665–66 epidemic was on a far smaller scale than the earlier Black Death pandemic; it was remembered afterwards as the "great" plague mainly because it was the last widespread outbreak of bubonic plague in England during the 400-year Second Pandemic.