Parts of a national forest in California have been evacuated and closed down after a squirrel was found to be infected with the plague.
Los Angeles officials say visitors were ordered to leave the Angeles National Forest as a precaution after the rodent was trapped in a routine check.
They said no people in the area were believed to have been infected with the disease, known as the Black Death.
The plague killed as many as 25 million Europeans during the Middle Ages.
It is a bacterial infection which can be transmitted to humans through the bites of infected fleas.
If not treated with antibiotics, it is usually deadly.
There have been only four cases of human plague in Los Angeles County since 1984, none of which was fatal, according to officials.
Further testing of squirrels in the region will be carried out before the campgrounds are re-opened to the public.
Jonathan Fielding, director of the Los Angeles County Department of Public Health, told the BBC that agriculture workers would dust squirrel burrows in order to reduce the flea population.
He said that while the area was closed to camping, people would still be able to hike through.
He advised that anyone who wished to do so should use insect repellent and ensure that any pets they bring have a flea collar.