The Great Plague was a massive outbreak of disease in Britain that killed up to a fifth of London's population in 1665. It is generally believed to have been bubonic plague, an infection by the bacterium Yersinia pestis transmitted via a rat vector. Other infectious agents have also been suggested.
Though concentrated in London, the outbreak affected other areas of the country. Perhaps the most famous example was the village of Eyam in Derbyshire. The plague arrived in a parcel of cloth sent from London. The villagers imposed a quarantine on themselves to stop the further spread of the disease. Though successful, the village lost around 80% of its inhabitants.
The 1665 epidemic was in fact on a far smaller scale than the earlier "Black Death", a virulent outbreak of disease in Europe between 1347 and 1353, but was remembered afterwards as the "great" plague because it was one of the last widespread outbreaks in Europe.