Bubonic Plague

Yersinia pestis is an intracellular parasite. In contrast to other kinds of bacteria, it enters cells. The bacterium that causes tuberculosis is also an intracellular parasite, as is the bacterium that causes chlamydia, a sexually transmitted disease.

Humans are not the "first choice" of host for Yersinia pestis. The Yersinia pestis bacterium infects the bloodstream of rats and other wild rodents such as squirrels and prairie dogs. Humans become infected only through the bite of a flea that has ingested blood from an infected rodent. Another route of transmission is through person-to-person contact. If a person's lungs are infected with the bacteria, the disease can be transmitted easily to another person through a cough or a sneeze. This form of transmission is extremely quick: cases have been recorded of persons dying from the disease within 24 hours of exposure to an infected person.