What was the virus behind 'The Great Plague' of London?

What exactly happened during the Great Plague of London to kill 100,000, or a quarter of the population, just 350 years ago?

Daniel Defoe had something quite ghastly to say about it, even in the 18th century, in A Journal of the Plague Year, he wrote:

"The plague, as I suppose all distempers do, operated in a different manner on differing constitutions; some were immediately overwhelmed with it, and it came to violent fevers, vomitings, insufferable headaches, pains in the back, and so up to ravings and ragings with those pains."

Scientists investigated 20 of the 3,500 skeletons in a mass grave discovered last year during a Crossrail dig at the Bedlam burial ground, also known as the New Churchyard, in East London.

The skeletons belonged to the bodies that had been annihilated in 1665, during the second plague epidemic. They showed some bacterium that also caused the 1348 Black Death outbreak, which was the first plague epidemic to hit Britain.

"It's significant because we had this famous, severe outbreak of plague in 1665, but until very recently, there was quite a lot of doubt about what had caused it," a researcher told the Independent.

Hence, the bubonic plague bacteria Yersinia pestis led to the Black Death first, but did it also cause the Great Plague in 1665? At first, they thought that it had an effect that was different from the first outbreak.

Scientists extracted tooth pulp from five of the 20 patients who were examined, though. It at last set the matter to rest. "We could clearly find preserved DNA signatures in the DNA extract we made from the pulp chamber and from that we were able to determine that yersinia pestis was circulating in that individual at the time of death," a researcher says.

So its clear that Yersinia pestis was the villain and can be quite a killer when it is let loose.

What about you? You don't have to worry about plague of any sort---great or small---if you hop into London. Improved hygiene and antibiotics show that the risk for you is very low, says the Londonist.