The flea's bad reputation may be somewhat undeserved, according to new research. In the 14th century, the insect played a major role in the spread of bubonic plague, which killed tens of millions of people in Asia, Europe, and Africa. But the results of a new study published in the journal Cell suggest that plague was present in human populations twice as long as previously thought, and that it was initially spread by human-to-human contact.

The bacteria that causes plague,Yersinia pestis, was recently found in human teeth dating from 2800 to 5000 years ago. In studying the DNA, researchers found that while the bacteria could not have led to bubonic plague, specifically, it could have caused pneumonic and septicemic plague, resulting in significant population declines in the 4th and 3rd millennia BCE. "By sequencing the genomes, we find that these ancient plague strains are basal to all known Yersinia pestis," the researchers write. "Our findings suggest that the virulent, flea-borne Y. pestis strain that caused the historic bubonic plague pandemics evolved from a less pathogenic Y. pestis lineage infecting human populations long before recorded evidence of plague outbreaks."

The researchers believe that the older strand of Yersinia pestis could not have caused bubonic plague because six of the seven samples were missing key components found in modern samples. A University of Cambridge press statement explains the significance of the missing ymt gene and a mutation of the pla gene in the ancient samples: 

The ymt gene protects the bacteria from being destroyed by the toxins in flea guts, so that it multiplies, choking the flea’s digestive tract. This causes the starving flea to frantically bite anything it can, and, in doing so, spread the plague. The mutation in the pla gene allows Y. pestis bacteria to spread across different tissues, turning the localized lung infection of pneumonic plague into one of the blood and lymph nodes. 

In other words: Fleas likely had nothing to do with the spread of earlier plagues, because back then, the bacteria hadn't evolved the traits it needed to survive inside fleas' digestive tracts. What's more, thanks to a genetic mutation, modern-day Y. pestis is far more likely to have systemic effects than its prehistoric counterpart.