Y. pestis

Yersinia pestis is a Gram-negative, nonmotile, rod-shaped coccobacillus, with no spores. It is a facultative anaerobic organism that can infect humans via the oriental rat flea. It causes the disease plague, which takes three main forms: pneumonic, septicemic, and bubonic plagues. All three forms were responsible for a number of high-mortality epidemics throughout human history. In the urban and sylvatic (forest) cycles of Y. pestis, most of the spreading occurs between rodents and fleas. In the sylvatic cycle, the rodent is wild, but in the urban cycle, the rodent is primarily the brown rat. In addition, Y. pestis can spread from the urban environment and back. Transmission to humans is usually through the bite of infected fleas. If the disease has progressed to the pneumonic form, humans can spread the bacterium to others by coughing, vomiting, and possibly sneezing.