Since the reign of Emperor Justinian in 542 A.D., man has one unwelcome organism along for the ride, Yersinia pestis. This is the bacterium more commonly know as the Black Death, the plague. Plague is divided into three biotypes, each associated with one of three major pandemics occurring in history. Each of these biotypes are then divided into three distinct types, classified by method of infection.
The most widely know is bubonic, an infection of plague that resides in the lymph nodes, causing them to swell. The Black Death of the 14th century was mainly of this type. Bubonic plague is commonly spread through fleas that have made a meal from an infected Rattus rattus.
The most dangerous type of plague is pneumonic. It can be spread through aerosol droplets released through coughs, sneezes, or through fluid contact. It may also become a secondary result of a case of untreated bubonic or septicemic plague. Although not as common as the bubonic strain, it is more d...