The 1665 plague

 The worst fear of medevial people was the plague. There were a lot of them throughout the centuries but the one that will always remain in London's memory will definately be the plague from 1665.
  That epidemic was so tragic mostly because of two factors. The first was the amount of the dead during that plague: It was over 100.000 people, which at that time was 1/3 of the total population. The second was the wrong assumption that the Great Fire (1666) had cleansed the city of the disease.
 Some people believe that the epidemic was caused by black rats living in London, but there's also a theory that it was the water that carried the disease. But no matter what caused the plague, the river definately didn't get rid of it. 
 Only wealthy people could afford running away from London at that time (it was almost 20 % of London population). Thanks to one person that stayed in London during the 1665 plague we know how it all looked like. It was Samuel Pepys who wrote about the epidemic in his journal. This is a fragment from "The journal of the plague, year 1665" :

"Lord! how empty the streets are and melancholy, so many poor sick people in the streets full of sores; and so many sad stories overheard as I walk, everybody talking of this dead, and that man sick, and so many in this place, and so many in that."

source:
http://www.history.co.uk/study-topics/history-of-london/samuel-pepys-and-the-1665-plague