A woman living in La Plata County tested positive for bubonic plague on Friday.
It's the first case of bubonic plague in the county since 2006, according to the Centers for Disease Control and Prevention.
Claire Macpherson, spokeswoman for San Juan Basin Health Department, said the woman was hospitalized with plague-like symptoms on Saturday.
Classical symptoms include the sudden onset of fever, headache, chills, weakness and one or more swollen and tender painful lymph nodes; these are called "buboes," giving bubonic plague its name.
Doctors sent blood samples to the Colorado Department of Health and Environment for testing. The state confirmed that they had identified bubonic plague in her blood culture on Friday.
The woman has since been treated with antibiotics and released from the hospital.
The woman was never contagious, Macpherson said, because humans cannot catch bubonic plague from other humans. Typically, infected animals transmit the disease to people.
San Juan Basin Health is investigating how the woman became infected.
According to the CDC, in modern America, the threat posed to humans by bubonic plague has receded since the Black Death killed 75 million to 200 million people in the 1300s, wiping out as much as 60 percent of Europe's entire population.
For the last few decades, on average, bubonic plague has infected seven humans every year nationwide, Machpherson said.
The majority of cases tended to be scattered across rural areas and clustered in two regions: northern New Mexico, northern Arizona and southern Colorado; California, southern Oregon and far West Nevada.
Macpherson said "bubonic plague is endemic to our area, Southwest Colorado."
Plague-ridden fleas hold the greatest danger for humans.
Public health officials are warning locals to take precautions against the disease.
Though it's summer, Macpherson urged people going outside to protect themselves by wearing long pants, long sleeves and a layer of bug spray that is at least 30 percent DEET - especially if you're going into areas where there are prairie dogs.
She said it is critical that dog owners make sure their pets are free of fleas.
Also, don't let your pets sleep in bed with you. Research shows that inter-species bed-sharing increases your risk of getting plague, according to San Juan Basin Health Department.
Macpherson also emphasized the danger of approaching wild animals and cautioned people to eliminate everything in their yards, garages and driveways that is congenial to rodent life, including piles of lumber, broken cement, trash and weeds.
In 2012, a 7-year-old girl in Pagosa Springs showed up at the emergency room with flu-like symptoms that progressed to a 107-degree fever and seizures. After being flown to a Denver hospital, she tested positive for bubonic plague. The child was treated with a specific antibiotic regime and survived.
So far this year in Colorado, four other cases of human plague have been identified in Adams County. Two rodents and a dog have tested positive for plague. A dozen flea samples have also tested positive for plague, according to San Juan Basin Health Department.
Since 1957, Colorado has seen a total of 64 cases of human plague: In nine cases - 14 percent - the disease proved fatal.
In humans, symptoms begin two to six days after being bitten by a plague-infected flea or contact with an infected rodent or cat.