There were many incorrect theories about the cause of the bubonic plague.
One of the most common was the idea that it was caused by bad air. This led some people to breathe through masks or little sacks of scented materials (these sacks were known as posies, thus giving rise to "A pocket full of posies" in our "Ring Around the Rosie" nursery rhyme which is based on the Black Death).
Another common belief was that the Plague was sent by God as a punishment for the sins of mankind.
A third belief held that the Jews had caused the disease as a way of trying to kill Christians.
Finally, there were some who believed in an astrological cause -- that an unlucky alignment of heavenly bodies had caused the black death.
Many Europeans believed that the Black Death was spread through breathing unhealthy air, so those who could afford it left London and other cities and waited out epidemics at their country homes.
They credited the fresh rural air with protecting them from the plague, when in fact it was removing themselves from the proximity of those people whose homes and workplaces were infested with rats carrying fleas that carried the infection.
Ironically, the country rats were just as infested with fleas, but those fleas did not carry the plague because the country rats did not stow away on ships, like the city rats did.
Bubonic plague has been a major epidemic in the world since ancient times. There is a record of plague epidemic in Athens in 430 B.C. In a plague epidemic in Rome in 262 A.D. nearly 5,000 people died daily.
I am not aware about different theories that may have circulated in the past about cause of bubonic plague. However, in 1907 it was well proven that rat fleas carry this disease, and since then it has been possible to control it with campaigns to control rats.
Bubonic and other forms of plague is caused by germs called yersenina pesis which is transmitted to human beings chiefly by rat fleas from infected rats. The rats used to spread plague from place to place in ships and trains. Once human beings are infected with plague, they can infect other people also.
1. Bad air caused the plague.
2. Punishment from the God
3. Jews caused the plague to eliminate Christianity
4. Astrological reasons.