Plague is a severe disease caused by the bacteria, Yersinia pestis. It has several forms - bubonic plague, pneumonic plague, and septicemic plague. It can be successfully treated with antibiotics if the disease is recognized in time.
This is the disease that caused the "black death" throughout history. It is transmitted by fleas from infected rats and other rodents. It can also spread person to person via body fluids and aerosol droplets.