Plague is caused by the bacterium Yersinia pestis and is transmitted between rodents and humans via flea vectors. Rodents like rats and gerbils serve as reservoirs for the plague bacterium. Fleas become infected by feeding on infected rodents and transmit the bacterium to other rodents or humans during subsequent blood meals when they attempt to feed but cannot due to blockages in their digestive tract. There are three main forms of plague - bubonic, septicemic, and pneumonic - with pneumonic plague being able to spread from person to person. Historically, plague pandemics have contributed to major population declines and changes in societies.