Zoo officials learned late Friday that the 8-year-old animal that died Wednesday tested positive for the flea-borne disease, according to a zoo statement.
More than a dozen squirrels and at least one rabbit have been found dead of plague in the City Park area just east of downtown, which includes a golf course, the zoo and the Denver Museum of Nature of Science.
The monkey, which was acting lethargic, was found dead by a zookeeper. Zoo veterinarian Dr. David Kenny suspects the primate ate the carcass of an infected dead squirrel.
Plague is common in Colorado during this time of year, but it usually occurs in rural parts of the state, where it's sometimes discovered when entire prairie dog colonies die off.
"We see it every year in wild rodents," said state health department epidemiologist John Pape. "But it's uncommon circulating in tree squirrels in urban neighborhoods, including metro Denver."
No other animals, including the other 17 capuchins, have shown any signs of illness, but as a precaution the monkeys have been taken from their usual display and placed in a separate caged display for observations. The capuchins have also been given antibiotics as a preventive measure.
Kenny said zoo officials are considering other measures on how to better shield other animals from possible contact with squirrels.
"It's a first time occurrence at the zoo, as far as we know," said Kenny.
The chances that a human could be infected is considered minimal. Health department officials suspect the disease was brought into the city by a fox or coyote.
One form of the plague, bubonic, is believed to have been the "Black Death" that killed 25 million people in Europe during the Middle Ages.