The squirrel, trapped July 16, tested positive for the infection on Tuesday, prompting a public health advisory. The campground will remain closed while investigators test other squirrels in the area and dust for infected fleas.
"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," L.A. County health officer Dr. Jonathan Fielding said in a statement.
Plague is caused by Yersinia pestis, and although it wiped out more than half the European population in the fourteenth and fifteenth centuries, the Centers for Disease Control and Prevention report an average of seven human cases per year in the U.S.
Modern plague cases typically appear in the rural West, and plague has been known to reside in the San Gabriel Mountains ground squirrel population.
Plague transmission through flea bites causes bubonic plague, with symptoms including rapid onset of fever and chills and enlarged lymph glands near the bite. Untreated bubonic plague can progress to infection of the blood or lungs, causing pneumonic plague. All forms of the plague can be fatal if left untreated.