The U.S. went a decade without any cases of the plague, but this summer the illness has made headlines after several people contracted the disease while visiting Yosemite National Park.
On August 18, California officials announced that they were looking into a case of plague in a Georgia resident who visited the park this month. The patient is presumed to have the plague, but the CDC has not yet confirmed the case. 
Last month, a Los Angeles county girl was also diagnosed with plague after visiting Yosemite. Health officials say she is recovering.
Experts haven’t pinpointed the exact cause of the sudden appearance of plague, but many believe the behavior of people and rodents, or even the California drought, could be to blame. The last time someone was diagnosed with the plague in that state was in 2006. 
The plague is rare in California, but not unheard of. Dr. Amesh Adalja, an infectious-disease specialist and a senior associate at the University of Pittsburgh Medical Center’s Center for Health Security, said rodents in California and other Western states are known to carry fleas that can transmit the disease.
“These cases are occurring in locations where we know that plague exists in the [rodent] population,” Adalja said.
The risk of plague is likely increased by people feedings rodents and other animals carrying plague-infested fleas, or by staying in cabins built in a new area, as rodents easily make their way into campgrounds.
Past studies point to weather as a possible contributor to the rise of plague, though no research has focused specifically on the California drought. Dr. Bruno Chomel, a professor at the University of California at Davis, School of Veterinary Medicine, says the drought could be making it more difficult for rodents to find food nutrition sources, so they are overrunning campgrounds in a hunt for food. Combine that with warm, dry weather that is favorable for flea activity, and you have a breeding ground for plague.
Plague transmission is also relatively dependent on the type of rodents that are in a given area. Plague rarely makes smaller rodents such as mice severely ill, but squirrels and chipmunks that become infected with the disease usually die. If plague-infested fleas hop from small rodents to squirrels, for example, and the larger animals subsequently die, the fleas will move on to new hosts.
Plague is caused by bacteria called Yersinia pestis. The disease is famous for causing the Black Death in Europe during the 1300s. An estimated 75 million people – between 30 and 50 percent of Europe’s population – died during that time.
There are three types of plague – bubonic, septicemic and pneumonic. The bubonic plague is the most common type of the disease, the symptoms of which include swollen lymph nodes that can be the size of an egg that are hot to the touch in the groin, armpit or neck; sudden onset of fever and chills; headache; fatigue or malaise and muscle aches. 
So far this year, there have been two deaths from plague in Colorado, in addition to the two California cases.
 Fox News
 Mayo Clinic