A park in Los Angeles has been evacuated and shut down after a squirrel was found to be infected with the plague.
The squirrel tested positive for the plague after it was trapped in the National Forest during routine surveillance activities.
Los Angeles County Department of Public Health said it did not believe any humans have been infected with the disease.
The plague was one of the most devastating pandemics recorded in human history and is believed to be responsible for the deaths of up to 200 million people during outbreaks in the 14th, 17th and 19th centuries. It spread through bites of infected fleas.
Also known as the Black Death, symptoms include swollen nymph nodes on the groin, neck and armpits, which oozed puss and blood.
Heath officials ordered the evacuation of the park in LA, covering 655,000 acres, as a precaution.
Jonathan Fielding, a health department spokesman, said: "Plague is a bacterial infection that can be transmitted to humans through the bites of infected fleas, which is why we close affected campgrounds and recreational areas as a precaution while preventive measures are taken to control the flea population.
"It is important for the public to know that there have only been four cases of human plague in Los Angeles County residents since 1984, none of which were fatal."
Previous surveillance identified another squirrel with the plague in 2010, another in 2007, two in 1996 and one in 1995.
Further testing of squirrels and rodents in the area will be carried out and the park will be dusted for fleas before it is reopened.
Humans infected with the plague now generally respond well to treatment with antibiotics.
Fielding advised anyone taking part in outdoor activities around LA county should use insect repellent. An advisory note by the health department said: "Transmission of plague through flea bites causes bubonic plague, with symptoms including enlargement of lymph glands (buboes) near the flea bite and rapid onset of fever and chills.
"Untreated bubonic plague can progress to infection of the blood, or rarely, the lungs, causing pneumonic plague."