Workers with the Colorado Health Department tested a carcass that was collected on Wednesday and preliminary results were positive for plague.
The plague can be spread to humans by fleas or by direct contact with infected animals, such as rodents, rabbits and cats.
"Typically the symptoms will develop in about two to three days - include a really high fever, lethargy, malaise - I mean you're sick - you're can't-get-out-of-your-bed sick," said John Pape, a state epidemiologist.
Plague is found among rodent populations in rural areas of Colorado every year, but experts say it is unusual to find the disease in the center of a city.
Although the illness is potentially life threatening, human cases are rare and plague can be easily treated with common antibiotics when recognized early.
According to state health officials, there have been 58 cases of human plague since 1957 and nine were fatal. Colorado's last death from plague occurred in 2004.
Click here for additional information from the state health department. You can contact the Colorado Education Line for the Public (CO-HELP) at 1-877-462-2911 to report a dead rodent or rabbit.