Definition of "pestis" [pestis]

• A serious (sometimes fatal) infection of rodents caused by Yersinia pestis and accidentally transmitted to humans by the bite of a flea that has bitten an infected animal (noun)

WordNet. Princeton University. 2010.

• "In ancient and medieval times, people thought the plague was caused by rats, but the true cause wasn't discovered until 1894, when Alexandre Yersin of France and Kitasato Shibasaburo of Japan finally traced it to a bacterium now called Yersinia pestis, which is transmitted by fleas, which in turn are carried around by rats."
• "Another plague that really made the “dark” Middle Ages dark was the Black Death, or bubonic plague, which is caused by the bacterium Yersinia Pasteurella pestis."
• "* It was widely caused by a bacterium named Yersinia pestis and was spread by rats and fleas."