HOW DID THE 1665 PLAGUE START AND SPREAD? Source: The Great Plague and Fire of London by Charles J Shields, Chelsea House Publishers. 2002.
Rats and Fleas
Doctors didn't know this at the time, but rats caused of the disease. However, the disease wasn't spread by the rats themselves, but by the fleas on their backs carried the disease.
Sea Port Location
In the early summer of 1665, infected rats escaped a Dutch ship and entered London.
Poor Sanitation
Sixty years preceding the outbreak, the population of London had more than doubled. The sheer number of people living in London were making the already filthy streets, ditches, and waterways even more polluted.
Hot Weather
Since it was summer, the weather was hot. The hot weather caused the fleas on the rats to multiply and reproduce, causing the disease to spread even more throughout the city.