When the plague broke out in the slums of the city it was ignored at first because there had been sporadic outbreaks of the disease before. When people realised that they were facing a major epidemic it
was already too late. Everyone who could afford it fled to the countryside. Even the doctors deserted their posts in the hospitals and left it to the Quakers to take care of the dying masses.
In a mad attempt to stop the plague from spreading further, all 40,000 dogs and 200,000 cats in the city were killed. (I bet the rats were pretty happy about that.)
During the unusually hot summer, up to 6,000 Londoners succumbed to the plague every week.
All in all, 100,000 - 200,000 people died.