Cougars in the United States have been contracting the same bacteria that causes the disease that killed a number of the people throughout the world in the mid-14th century, the bubonic plague. Researchers studying cougars (Puma concolor) in the Greater Yellowstone Ecosystem found that about 50% of the big cats, both living and dead, tested positive for plague antibodies in their blood. The bacteria that carries the plague, Yersinia pestis, lives in the soil and infects small hosts like fleas that can then spread the bacteria to rodents. Researchers believe the cougars contracted the plague from eating rodents or animals that eat rodents. The disease isn’t necessarily fatal in cougars, and some have clearly recovered from infection.