Investigators believe the victim may have developed the rare disease after coming into contact with the fleas on a dead rodent.
An adult in Pueblo County has become the second person to die from the plague in Colorado this year.
Health officials believe the unnamed individual may have contracted the disease from the fleas on a dead rodent.
Cases of the plague in humans are exceptionally rare in the US, with about seven reported each year on average.
However, eight cases were reported in Colorado alone during 2014.