What did they believe were the causes of the plague ?

In 1665 people were very catholic. They believed the plague had been sent by God as a punishment.

 At this time they had poor sewer systems, they were just mere streams of human waste in the streets. They believed that the bad smell from these sewers was infecting them with the deadly disease. 

Some people thought that the coming together of planets was the cause. Many people think that when planets aline, it means they is bad things to come. 

Some people thought the Black Death was caused by invisible fumes or poisons that could kill people from either looking at each other or merely breathing air in.