Parts of a Colorado wildlife refuge remained closed off on Sunday after officials first discovered plague-infected prairie dogs there in late July. Wildlife and nature areas near Denver have been shut down as officials continue efforts to stem the spread of the disease.

Rocky Mountain Arsenal National Wildlife Refuge, a 15,000 acre nature area northeast of Denver, was partly reopened on Sunday. The refuge is home to many species, including bison and bald eagles, and where the plague concerns first developed in the black-tailed prairie dog.

Plague-infected fleas were biting the prairie dogs, and officials began closing affected areas as a precautionary measure to protect visitors' health and safety, and also protect wildlife health, the U.S. Fish and Wildlife Service said.

Certain areas remain closed because of the risk posed by hiking with pets. Dogs are less susceptible to plague than cats, but may pick up fleas that can infect other animals and people, said Gilbert Cazier, a county environmental health specialist.

"If you bring the dog home and he sleeps in your bed, those fleas can then jump and get onto you," Cazier said.

Though the plague can now be treated with antibiotics, it has a dark history, and according to the Centers for Disease Control, was responsible for the death of 60% of Europe's population during the Black Death. In 1900, rat-infested ships sailing from areas with the plague led to epidemics in U.S. port cities, but the last epidemic was in Los Angeles in the 1920s.