A squirrel infected with the plague has been discovered near Los Angeles, prompting US officials to close three campsites.
The diseased animal - a ground squirrel - was trapped in the Angeles National Forest during routine surveillance by the US Forestry Service.
Jonathan E Fielding, LA's director of public health, 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."
A health advisory issued by the County of Los Angeles Public Health department said plague has been known to reside in the ground squirrels population in the San Gabriel Mountains.
The advisory added that transmission of plague through flea bites causes bubonic plague in humans, with symptoms including enlargement of lymph glands (buboes) near the flea bite and rapid onset of fever and chills.
Known as the Black Death, bubonic plague killed around 25 million people across Europe in the 14th century.
All forms of the disease can be fatal, but most patients respond well to antibiotics.
Previous routine surveillance had identified one plague-positive ground squirrel in 2010, one in 2007, two in 1996 and one in 1995.
Officials say further testing of the rodents will be carried out and squirrel burrows in the area will be dusted for fleas before the area is re-opened to the public.
Campers at the affected sites - Broken Blade, Twisted Arrow, and Pima Loops - have been informed.