English: Figure 1. Y. pestis infection can lead to three forms of plague. Bubonic plague can result from flea bites or handling infected animals and can result in a severe localized infected lymph node referred to as a buboe (green). Septicemic plague can result as a secondary issue arising from bubonic disease or if the bacteria are introduced directly to the blood stream (red). Pneumonic plague is the most severe form of the disease and can result in person-to-person spread if untreated and initiated by the deposition of Y. pestis bacteria into the respiratory tract of individuals (blue).