The plague is one of the deadliest bacterial infections in human history. An estimated 50 million Europeans died from it in the mid-1300s when it was known as the “Black Death.”

Thanks to improved sanitation, public health measures and antibiotics, the Centers for Disease Control and Prevention now reports an average of only seven human cases of plague in the U.S. each year. While humans are largely safe from it, particularly in the eastern half of the U.S., the disease continues to wipe out prairie dog colonies and infect other rodents in the western half of the U.S.