Los Angeles-area campgrounds closed after squirrel found with the Plague, known as the "Black Death"

Authorities have evacuated and shut down a section of a national forest outside Los Angeles for at least a week after a ground squirrel was found there infected with the plague, county public health officials said on Thursday.

The squirrel tested positive for plague after it was trapped in the Angeles National Forest during "routine surveillance activities," the Los Angeles County Department of Public Health said in a written health advisory.

Health officials said that as a precaution, visitors were ordered to leave three campgrounds and a recreation area of the Angeles National Forest, which encompasses some 655,000 acres in the San Gabriel Mountains just north of metropolitan Los Angeles.

Plague, known as the "Black Death" when it was blamed for killing some 25 million Europeans during the Middle Ages, is a bacterial infection that can be transmitted to humans through the bites of infected fleas.

A health department spokesman said no people were believed to have been infected.

"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," the health department chief, Dr. Jonathan Fielding, said in a statement.

An average of seven cases of plague are reported each year in the United States, according to the U.S. Centers for Disease Control and Prevention, mostly in western states. The disease is typically not fatal if treated with antibiotics.

Officials said further testing of squirrels would be conducted in the Angeles National Forest before the Broken Blade, Twisted Arrow and Pima Loops campgrounds were re-opened to the public.

According to the health department, plague has been known to reside in the San Gabriel Mountains ground squirrel population. Previous surveillance efforts have identified five other squirrels carrying the disease since 1996.

The most recent squirrel found to be infected was trapped on July 16 during routine checks in the park and confirmed to carry the disease on July 23, according to the health advisory.

Last year, an Oregon man survived a bout with plague, one of three reported in that state since the mid-1990s, none of which were fatal.