Denver officials warned people to stay away from squirrels as they monitored what could be an outbreak of the plague.
Fifteen squirrels and one rabbit have tested positive for the plague, which is typically transmitted by flea bites, The Denver Post reported. Most of the animals were from the Denver City Park area.
Denver has not had an outbreak of the plague since 1968.
The state Health Department set up a hotline for people to call about dead squirrels, and the officials in Boulder closed a park to test a prairie dog town, the Post reported.
"We're still trying to determine how widespread it is," state epidemiologist John Pape told the Post.
"I wouldn't apply the term 'outbreak' to this," he told the newspaper. "We normally see this kind of rodent die-off where plague is occurring, and it just happens to be occurring in Denver this year."
The Post said Colorado is one of a handful of states where the plague bacterium, which killed 25 million Europeans in the Middle Ages, occurs.
The plague kills 1,000 to 2,000 people every year worldwide, including about 18 in the United States.
Copyright 2007 by United Press International
Explore further: Experimental Ebola drug heals all monkeys in study (Update)