Yersinia pestis is a bacterium that causes the infectious disease known as the plague. It is primarily transmitted to humans through bites from infected fleas, which often live on rodents like rats. The bacterium can also spread through direct contact with infected animals or through respiratory droplets from an infected person. There are three main forms of the plague: bubonic plague, characterized by swollen lymph nodes; septicemic plague, which affects the bloodstream; and pneumonic plague, which infects the lungs. If left untreated, the plague can be deadly, but antibiotics are effective in treating the infection if administered promptly.