In 1665 an epidemic spread all over London which killed more than 100,000 people in the city within 18 month. It was a significant proportion of the population because at that time only 460,000 people lived in London.
The great plague of London was a bacterial infection which was caused by fleas which took the Yersinia pestis bacteria from rats and infected people with it, it was not the first infection from rats in London because previously more server epidemic caused more deaths.
As London was a dirty city with no proper sanitation and open drains which ran along side streets there were rats almost everywhere. During summer flies and stinking smell of garbage in the streets was a common thing, these factors caused the great plague.
Such plagues killed millions around the world, especially in Europe, China & India where people either lived together or interacted more.