The plague is generally carried by fleas, though how exactly the girl came to be infected is still under investigation.
“We are so sad for the loss of this young Coloradan and our deepest condolences go to the family,” Dr Jennifer House, Colorado state veterinarian, told the Associated Press.
She urged residents to pay attention to symptoms and seek medical help if they believe they might have the plague.
“[Denver] Public Health is doing an epidemiological investigation and wants Coloradans to know that while this disease is very rare, it does occur sometimes, and to seek medical care if you have symptoms,” she said.
Four people died of the plague in 2015, with half of that number coming from Colorado.