Plague is an infectious disease caused by the bacterium Yersinia pestis.

Plague has received attention because of its potential use as as biological weapon by terrorists. If intentionally released, the aerosols could cause pneumonic plague in affected populations.

It is important for medical personnel and first responders to prepare for such an event and to be aware of any risk of person-to-person infection.

The typical route of infection in humans is from a rodent flea carrying the plague bacterium or from handling an infected animal. In the Middle Ages, millions of people died in Europe from plague because human homes were inhabited by flea-infested rats. Though antibiotics are effective against plague today, if an infected person is not treated, the disease causes death.