Gram-negative straight rods, sometimes approaching a spherical shape. Y.pestis is always nonmotile. It is the causative agent of plague which is primarily a disease of wild rodents. Y.pestis is transmitted among wild rodents by fleas, in which the bacteria multiply and block the esophagus and the pharynx. The fleas regurgitate the bacteria when they take their next blood meal. Bacteria are transmitted subcutaneously to humans by the bite of infected fleas, but also by air, especially during pandemics of disease. Infective flea bites produce the typical bubonic form of plague in humans. Y.pestis is very closely related to the gastrointestinal pathogen Yersinia pseudotuberculosis, and it has been proposed that Y.pestis evolved from Y.pseudotuberculosis 1,500-20,000 years ago. Strain 91001 is avirulent in humans.