Since a woman in La Plata County tested positive for bubonic plague last week, the San Juan Basin Health Department investigators has raced to identify the origin of her disease.

The Health Department announced Thursday its team of investigators had discovered the likely source: a burrow of prairie dogs in western La Plata County ridden with plague-positive fleas.

Joe Fowler, an epidemiologist with San Juan Basin, said health officials could not name the burrow's location, citing the patient's right to privacy.

The last time a human came down with bubonic plague in La Plata County was 2006, according to the Centers for Disease Control and Prevention.

The woman was treated and released Aug. 8.

Whenever there are outbreaks of dangerous infectious diseases such as bubonic plague, smallpox and Ebola, containment only partially depends on infected patients' receiving adequate medical care. To prevent further transmission, public health officials also have to do old-fashioned detective work to ascertain the disease's etiology, tracing the path of infection it blazed - traveling from host to host, jumping between species - back to its earliest origin.

In a phone interview Thursday, Fowler said almost 10 San Juan Basin Health employees had worked to track down the vector of last week's report of bubonic plague with help from experts and laboratories at the Colorado Department of Health and Environment. Working with information provided by the plague patient, investigators eventually narrowed in on the burrow of plague positive, flea-infested prairie dogs as probable exposure source.

It's impossible right now to determine the number of infected prairie dogs, Fowler said.

But he said humans had to be cautious.

"The main message here is that in summertime in Colorado, plague can occur at any place and time. Plague is endemic to our area, and everybody in La Plata County - no matter where they live - has to take precautions," he said.

Since 1957, Colorado has seen a total of 64 cases of human plague: In nine cases - 14 percent - the disease proved fatal.

This year in Colorado, four other cases of human plague have been identified in Adams County. Two rodents, a dog and a dozen flea samples also have tested positive for plague, according to San Juan Basin Health Department.

According to Joe Lewandowski, of Durango Parks and Wildlife, prairie dogs have failed to develop any resistance to plague.

Bubonic plague regularly wipes out enitre colonies.