Health authorities have closed three campgrounds in a forest near Los Angeles after the discovery of a squirrel infected with the plague.

County officials and the US Forest Service shut the campgrounds in Angeles national forest on Wednesday after a trapped squirrel tested positive for the disease.

"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," Jonathan Fielding, head of the county health department, said in an advisory.

Bubonic plague was known as the black death when it devastated medieval Europe, killing millions, but is now rare, treatable with antibiotics and seldom fatal to humans. There have been just four cases of human plague in LA county residents since 1984, said Fielding. All survived.

The shuttered campgrounds, Twisted Arrow, Broken Blade and Pima Loops, are on Table Mountain, part of the San Gabriel mountains, near the town of Wrightwood.

The ground squirrel – part of a group so named for preferring to live on the ground rather than trees – was trapped for routine surveillance on July 16. The test result was confirmed on Tuesday.

Plague has been identified in the San Gabriel ground squirrel population before. Many in California regard them as pests.

Officials said they would dust the forest's squirrel burrows for fleas. They warned people to avoid visiting areas near the campgrounds, which are likely to remain closed for at least a week, and to not feed wild animals. They also said people should protect pets from fleas and use insect repellant.

"Protection with an insect repellent containing Deet is also recommended for persons visiting the Angeles National Forest and engaging in outside recreational activities in other areas of LA county," Fielding said. Products with Deet are not safe for pets.