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By Mayo Clinic staffPlague is a life-threatening infection caused by the organism Yersinia pestis. There are three types of plague. Bubonic plague is the most common type in humans. Infected fleas transmit Y. pestis primarily among rodents. When an outbreak kills many rodents, infected fleas can jump to other animals and humans, spreading the infection. Improved living conditions and health services have made human outbreaks uncommon, but occasional plague cases occur.
Concern exists about the use of plague as a biological weapon. Plague bacteria could be put into a form that might be sprayed through the air, infecting anyone inhaling it and causing pneumonic plague. This form affects your lungs and can spread from person to person.
Fortunately, when given promptly, antibiotics usually effectively treat plague.