The unnamed tourist from Georgia visited Yosemite and the Sierra National Forrest in early August.

Director of the California Department of Public Health, Dr Karen Smith, said: “Although this is a rare disease, and the current risk to humans is low, eliminating the fleas is the best way to protect the public from the disease”.

The U.S. Centers for Disease Control and Prevention says 1,006 probable human plague cases have been recorded between 1900 and 2012.

Health officials in California are now investigating another possible plague case in a Georgia tourist. Two other Coloradans who caught plague this year were treated and recovered, including one whose dog is suspected of having carried the infection.

The first Yosemite campground to get the once over was Crane Flat, which is where a child – after visiting nearby Stanislaus National Forest – had stayed in mid-July. Tuolumne Meadows Campground in Yosemite is closed this week for a similar treatment after infected squirrels were detected in the area. Animals infected with the plague are usually found in the mountains or foothills but can sometimes be found along the California coast.

A National Forest Service official said Wednesday that there are no signs that plague is spreading in the park, but warning signs have now been posted at places where the Lindquist family visited “out of an abundance of caution”. Since 1970, there have been 42 people infected with plague in California, with nine deaths.

The FDA approved the drug Levaquin in 2012 to treat the plague, joining other antibacterial drugs, such as streptomycin, doxycycline, and tetracycline, that are approved for the treatment of the infection. Officials believe the child came into contact with sick rodents, fleas, or mouse dust and droppings, all known to carry the plague. This means never feeding squirrels, chipmunks or other rodents, never touching sick or dead rodents, avoiding walking or camping near rodent burrows and keeping wild rodents out of homes, trailers and outbuildings and away from pets.

Buttke stressed that human cases of plague in Yosemite park are exceptionally rare with the last confirmed case occurring in 1959.

Plague symptoms do include fever, nausea, weakness, chills, and swollen lymph nodes in the armpit, groin, or neck area.

Despite its reputation as a mass killer of humans throughout history, plague – which wiped out about 30% of the European population in the 1300s – has become a vastly more reliable killer of wildlife than people.