There were several reasons for the dying out of the Black Death that occurred in London in 1665. Vast improvements in the sphere of healthcare, removal of the slums in London, and introducing the brown rat are some of the causes for the disappearance of the Black Death. Development of immunity against the disease also had a tremendous impact in its dying out.
How Many People Died from the Black Death? – http://answers.ask.com/Society/Other/how_many_people_died_from_the_black_death