Between April 1665 and September 1666 plague killed 75,000 to 100,000 people in London, up to a fifth of London's population.
"The disease was historically identified as bubonic plague, an infection by the bacterium Yersinia pestis, transmitted through a flea vector. The 1665-1666 epidemic was on a far smaller scale than the earlier "Black Death" pandemic, a virulent outbreak of disease in Europe between 1347 and 1353. The Bubonic Plague was only remembered afterwards as the "great" plague because it was one of the last widespread outbreaks in England.
"At the time, the outbreak was blamed upon the French. In early April 1665, two infected French sailors were said to have collapsed and died at the junction of Drury Lane and Long Acre in London. These cases were said to have brought about all subsequent infections. This theory has been largely dismissed as anti-French propaganda. The British outbreak is actually thought to have originated from the Netherlands, where the bubonic plague had occurred intermittently since 1599, with the initial contagion arriving with Dutch trading ships carrying bales of cotton from Amsterdam. The dock areas outside of London, including the parish of St. Giles-in-the Fields where poor workers crowded into ill-kept structures, were the first areas struck by the plague. Personal and public hygiene was very minimal during this period, contributing to the spread of disease. During the winter of 1664-1665, there were reports of several deaths. However, the very cold winter seemingly controlled the contagion. But spring and summer months were unusually warm and sunny, and the plague spread rapidly. As records were not kept on the deaths of the very poor, the first recorded case was a Rebecca Andrews, on April 12, 1665" (Wikipedia article on Great Plague of London, accessed 01-03-2009).