For centuries the Bacillus Yesinia Pestis, the bacteria associated with the plague, lived comfortably within the confines of the blood stream of the medieval wild black rat and the stomachs of adult rat fleas. For centuries, human populations were left untouched. The disease spreads primarily through the bite of these fleas. Rats are merely the vehicle for transporting the fleas from one area to another. Rats, like humans, eventually contract the disease and die. This forces diseased fleas to seek yet another warm-blooded mammal upon which to survive.
Despite that facts that the habitats of wild rats and humans rarely crossed paths and that rat fleas seldom found the blood of humans to be an enjoyable meal, people were occasionally the victims when no other warm-blooded mammals were available.
After a flea bites an infected rat and drinks blood with Yesinia Pestis, the bacteria begins to multiply in the flea's stomach. Once its gut becomes full of the bacteria, the flea must regurgitate some into a victim to make room for food in its stomach. It is at this point that the victim is infected. Here is a diagram explaining the process:
As the number of infected humans became more numerous, some survived and developed the disease in their lungs. Thus, the disease was no longer confined to the rat-flea-man cycle. In its pneumonic form, the plague could be transmitted through the air. This new form was much deadlier.
The roles of rats and fleas as vectors in the transmission of the disease was never suspected by the people of the time. There were several reasons for this. Both of these pests were very common at the time so they were not a source of suspicion. Also, the fact the plague was spread by respiratory transmission and infection further confused people. The medieval mind was simply not advanced enough to make the connection.
Several explanations were presented by different scholars of the time. Some chose to blame a recent earthquake, saying that it released some toxic gases. The scholars of the University of Paris concluded that the plague was a result of a celestial event that was occurring at the time. No one really believed these explanations. The common people had only one way to describe it -- that it was the wrath of God punishing them for their sins. The absence of an identifiable earthly cause gave the plague a supernatural and sinister quality.