The days of flea-infested rats spreading disease are not behind us in the Middle Ages. A new bacterium discovered in the past two decades may be taking up the mantle of "Plague" as it destroys hearts.

Twenty strains of Bartonella bacteria have been discovered since the early 1990s, though a strain that is the prime suspect in prosthetic heart valve infections was first described during World War I, according to the Centers for Disease Control, and was the suspected cause of 1 million cases of "trench fever".

Bartonella primarily target the heart, causing a damaging infection called endocarditis. Vegetations grow in the heart muscle causing valve dysfunction, decreased or strained pumping, and interrupted blood flow. It is a serious, and once relatively rare, heart disease caused most often by systemic infections with Staphylococcus aureus, the instigator of severe skin wound infections.

The strains appear to be carried by rodents, and of particular interest is the brown rat, the biggest and most common in Europe. Scientists are still evaluating the route of transmission, but studies so far indicate fleas to be the culprits, much like the Black Plague of Medieval Europe, which involved the transmission of the bacterium Yersinia pestis.

Bartonella strains can also infect the spleen and nervous system. A traveler to South America was unfortunate enough to provide a sample of the spleen infecting strain. But it is not only a European, South American, or travelers’ problem: The bacteria have also been found in patients in the United States and rats in Taiwan, indicating that a global problem is afoot.

The latest study finding the bacteria in several types of rodents and fleas is available in the December issue of Journal of Medical Microbiology.