A Los Angeles child was diagnosed with the plague after camping in Yosemite National Park in California in July.
The California Department of Public Health announced that an environmental evaluation would be conducted in Yosemite and neighbouring areas, including the Stanislaus National Forest.
Other members of the child's camping party were being monitored, but none of them had shown symptoms characteristic of the bacterial disease, which is normally spread through the bite of an infected flea or by coming in contact with a rodent that carried infected fleas.
''Human cases of plague are rare, with the last reported human infection in California occurring in 2006,'' said Dr Karen Smith, the director and state health officer of the California Department of Public Health (CDPH).
''Although this is a rare disease, people should protect themselves from infection by avoiding any contact with wild rodents. Never feed squirrels, chipmunks, or other rodents in picnic or campground areas, and never touch sick or dead rodents. Protect your pets from fleas and keep them away from wild animals,'' Smith added.
The plague claimed the lives of an adult and a teen earlier this year in Colorado in two unrelated incidents.
Though the plague killed millions of people in the 1300s, developments in modern medicine had halted widespread infection from the disease.
Plague is caused by the Yersinia pestis bacteria and spreads quickly via infected rodents, fleas, and sometimes people. Although it might be scary, in most cases current antibiotics are more than a match for the bacteria.
The earlier an infected person gets treated, the better, but from the symptoms, it presents people cannot make out that they have the plague. In the bubonic version, lymph glands swell, a much clearer sign than the coughing, fever, and weakness combination that is present in pneumonic plague.
Seven or eight plague cases are reported in the US annually, mostly in the southwestern states where rodents and fleas are still responsible for most of the transmission. While it was an obviously serious, potentially life-threatening illness, it was still rather rare.
Scientists at the University of Liverpool have found that outbreaks of plague in Madagascar are linked to the naturally occurring El Niņo climate event in the tropical Pacific. (See: Scientists show plague outbreaks linked to El Niņo climate conditions).