Officials at the Denver Zoo are scrambling to prevent a plague epidemic after a capuchin monkey was found dead of the disease.
All primates at the zoo are being isolated and treated with antibiotics.
More than a dozen squirrels and at least one rabbit have been found dead of plague in the area which includes a golf course, the zoo and the Denver Museum of Nature of Science. The monkey may have eaten 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.
No other zoo animals have shown any signs of illness and the chance that a human could be infected is considered minimal.
One form of the plague is believed to have been the "Black Death" that killed 25 million people in Europe during the Middle Ages