The person, whose name, age and gender weren't reported, tested positive for the rare illness, the Colorado Department of Public Health and Environment said.
The Colorado State University Veterinarian Diagnostic Laboratory conducted testing on the family dog after it was learned the animal died unexpectedly recently. The dog also tested positive for plague.
Health officials believe the person and dog contracted the disease in eastern Adams County.
The pneumonic plague can be spread from person to person through airborne droplets, whether through coughing or sneezing.
Pneumonic plague patients experience fever, headache, weakness and pneumonia with shortness of breath, chest pain, cough and sometimes bloody or watery mucous, the Centers for Disease Control website said.
This pneumonic plague is extremely rare. Some 999 plague cases were reported in the United States between 1900 and 2010, 80 percent of which are in the bubonic form.