The most common explanation of the transmission of the Black Death is flea bites. Infected fleas were carried on rodents, and these fleas transmitted the virus to humans.
There are still several different theories about how the Black Death was spread, however. There's a Wiki page called "Theories of the Black Death" that may be helpful.
At the time of the Black Death, bacteria and viruses hadn't been discovered yet. People thought that sickness was transferred through bad smells, which were called "miasma."
The long beaks of the masks were stuffed with strong-smelling herbs, spices, and dried flowers—lavender, roses, peppermint—which were thought to keep away the "evil" smells that were thought to cause disease.
Pretty cool stuff!
