Why pneumonic plague? Pneumonic plague is the form of infection that occurs when plague bacteria are inhaled in aerosolized form into the lungs, or when septicemic plague (plague in the bloodstreams) reaches the lungs. Like, for example, when a test tube breaks. Her method of infection was a pretty textbook vector for pneumonic plague. Other cases have been caused when tissue samples of infected rodents have been splayed open, for example, or when blood from an infected person splatters the respiratory tract. Plague bacteria spreads pretty poorly otherwise, which is why the main vector for bubonic plague is flea bites.