What is pneumonic plague, the disease linked to a reported outbreak in Russia?

A woman working at an anti-plague research facility reportedly died after breaking a test tube containing the pneumonic plague pathogen on September 25, following which 200 people were quarantined. Plague is caused by the Yersinia pestis bacterium and has three main forms, bubonic, pneumonic and septicemic. Symptoms include fever, headache, chills, weakness, shortness of breath and coughing.