Health officials from two counties in northern Arizona are warning the public that fleas are testing positive for the Yersinia pestis bacterium that causes plague.

Following a similar announcement from the Coconino County Public Health Services District, on Friday officials at Navajo County Public Health said that fleas there had also tested positive for the bacterium, ABC News reported.

"Navajo County Health Department is urging the public to take precautions to reduce their risk of exposure to this serious disease, which can be present in fleas, rodents, rabbits and predators that feed upon these animals," the public health warning stated. "The disease can be transmitted to humans and other animals by the bite of an infected flea or by direct contact with an infected animal."

Plague is the infamous flea-borne infection that killed millions of Europeans in the Middle Ages, decimating populations. The advent of better sanitation and modern antibiotics has held the disease largely in check, although small outbreaks can still occur.

Health officials are advising that people who live, work or visit the two Arizona counties avoid contact with sick or dead animals, and keep any pets from roaming loose, to minimize the chance of contact with rodents or fleas.

According to the U.S. Centers for Disease Control and Prevention, plague symptoms include the sudden onset of fever, chills, headache and weakness, as well as inflamed lymph nodes.