The bacterium Yersinia pestis has inflicted almost unimaginable misery upon humankind over the centuries, killing an estimated 200 million or more people and triggering horrific plagues in the 6th and 14th centuries.

But this germ was not always particularly dangerous. Scientists said on Tuesday minor genetic changes that it underwent many centuries ago — adding a single gene that subsequently mutated — turned it from mild to murderous.

Yersinia pestis caused two of the deadliest pandemics in human history: the sixth century Justinian Plague, named for the Byzantine emperor who was sickened but survived, and the 14th century Black Death. Rats with fleas carrying the germ spread the plague to people.