You probably don't need another reason—or any reasons other than good sense—to avoid getting too close to the rats that scurry around New York City's subways and garbage cans. But you can consider contracting diseases like the bubonic plague, also known as the Black Death, the pestilence icing on the avoidance cake.

In the first study of its kind since the 1920s (that's how long it took scientific curiosity to overcome the sheer ick-factor), scientists led by Matthew Frye, an urban entomologist with Cornell University's New York State Integrated Pest Management (IPM) Program, studied 133 rats and found more than 6500 specimens of five well-known species of fleas, lice, and mites, including 500-plus Oriental rat fleas, notorious for their role in transmitting the bubonic plague.

"If these rats carry fleas that could transmit the plague to people, then the pathogen itself is the only piece missing from the transmission cycle," Frye says. Fortunately, in the U.S. the plague pathogens are currently found only in the Southwest, where about 10 people a year are infected. But the plague isn't the only disease the researchers were concerned about.

"These pathogens can cause a wide range of clinical syndromes, some severe," says coauthor Cadhla Firth, a research scientist at Columbia University's Center for Infection and Immunity. To avoid these diseases, Firth advises not just rat-avoidance but careful sanitation of anywhere rats have been, since it's the parasites they may have left behind that you need to be wary of.

"It's not that these parasites can infest our bodies," Frye says, "but they can feed on us while seeking other rats to infest."

Ew.