When you think of disease-carrying animals, prairie dogs might not be at the top of your list, but as it turns out, they’re actually bringing a pretty nasty ailment to the Denver area. Fleas infected with the plague bacterium Yersinia pestis were discovered on prairie dogs in the Rocky Mountain Arsenal National Wildlife Refuge north of Denver late last month, and the park closed for several weeks. Although parts of the park reopened last weekend, other parts will remain closed through the summer.

According to John M. Douglas, Executive Director of the Tri-County Health Department, “the prairie dog colonies are being monitored and burrows are being treated with insecticide, but there is still evidence of fleas in the hiking and camping areas, which could put people and pets at risk, so those areas will remain closed.”

It might sound unlikely that modern-day prairie dogs have become infected by a 14th-century plague. Apparently, the bacteria had made its way to San Francisco around 1900, where it infected squirrels in the area before spreading across the American West. Although no humans have been impacted by the disease thus far, it can actually prove fatal to small mammals.

Paul Marinari, senior curator at the Smithsonian Conservation and Biology Institute, said, “it all depends on the species of prairie dog and the level of die-off. A number of prairie dog colonies see complete loss.”

For now, the Rocky Mountain Arsenal National Wildlife Refuge is doing everything it can to contain the outbreak and make the area safe for guests. “We are closing trails and spraying insecticide to kill fleas in plague-affected areas where there might be humans,” said the Refuge’s David Lucas. But then there is this secondary effort which is to try to prevent the spread of this disease across the landscape.”