Plague Cluster Reported in Colorado; Prairie Dogs Eyed

July 19, 2014— -- A plague cluster has been identified in Colorado after three residents contracted the disease, officials said.

There have been a total of four reported cases of the plague in Colorado so far.

"The Colorado Department of Public Health and Environment has identified three additional Colorado residents with plague, for a total of four cases. ... They all had mild symptoms, were treated with appropriate antibiotics, recovered, and no longer are contagious," Mark Salley, communications director for the Colorado Department of Public Health and Environment, said.

This is the first cluster of pneumonic plague in the United States since 1923, according to Dr. Richard Besser, chief health and medical editor for ABC News.

The three residents contracted the plague from a dog infected with the disease, according to Salley's statement. While human-to-human contraction is rare, the plague is most commonly spread by fleas from rodents, like prairie dogs.

Colorado health officials have been assessing prairie dog populations for plague bacteria, the statement said.