The plague -- aka the Black Death, aka the Great Pestilence -- is rarely contracted today, yet it recently infected a South Lake Tahoe resident. But before you put on your 17th century "air-purifying" beaked mask, let's find out from the experts why this disease is still around and how dangerous it is now.

Most people associate the term "plague" with the massive and destructive event that killed 25 million Europeans in the Middle Ages, said professor John Swartzberg of UC Berkeley's School of Public Health. In the 14th century, the plague killed as much as 50% of Europe's population.

"The plague is really a specific disease that has ... in our human history [reared] its ugly head and caused massive deaths," said Swartzberg, who works in the university's Infectious Diseases and Vaccinology division.

It may be a threat of the past, but it continues to be a very serious disease, with a small number of cases every year.

In the recent case of the South Lake Tahoe resident, it's believed the person contracted the rare disease after being bitten by an infected flea while camping in the area, according to El Dorado County health officials.

The most recent case before this one was reported in El Dorado County in 2020 and also was believed to have been transmitted in the same area, officials said. Two plague cases were reported in California in 2015, suspected to have been caused by bites from an infected flea or rodent in Yosemite National Park.

Even though medical advancements have made the deadly disease treatable, it hasn't been eliminated.

The plague is caused by the bacterium Yersinia pestis and primarily affects small animals or rodents in the United States, Swartzberg said.

There are three types of the plague:

It can spread to humans and pets from the bite of an infected flea or from coming into close contact with, or handling, an infected rodent.

The plague was introduced to the U.S. in the early 20th century from rat-infested steamships arriving in California from Asia, with the first case identified in the San Francisco area, Swartzberg said.

To date, the last known case of rat-associated plague occurred in Los Angeles in the 1920s, according to county public health officials.

The principal source of plague in Los Angeles County today is wild rodents in rural areas.

Most people who do contract the plague do so while hiking or camping, and in these areas they are unknowingly bitten by an infected flea, said Ashok Chopra, professor of microbiology and immunology at the University of Texas Medical Branch.

Animals live in much greater concentrations in rural areas, where they are in closer proximity to other creatures and habitats, making it easier for the bacterium to thrive, Swartzberg said.

Woodland creatures that could carry the disease and should be avoided include chipmunks, wood rats, mice and marmots.

In California, public health officials say the plague occurs in the foothills, plateaus, mountains and along the coast. It's largely absent from the southeastern desert region and the Central Valley.

There is the potential for exposure throughout Los Angeles County, but areas where the risk is highest are the rural recreational and wilderness areas of the Angeles National Forest and San Gabriel Mountains, county officials said.

People and their pets can get the plague if they visit or live in areas where wild rodents are naturally infected, according to the Los Angeles County Public Health Department. People who live in close contact with rats also are at greater risk of getting the disease from the possibly infected rodents.

It isn't uncommon for dogs and cats to become infected because pets do get fleas. An infected flea, although rare, can sicken pets, which are vulnerable to the disease, Chopra said.

The U.S. has eradicated only one human infectious disease, which is smallpox, and that was done through vaccination, according to the Centers for Disease Control and Prevention.

There have been several vaccine candidates to ward off the plague, but at this time there aren't any Food and Drug Administration-approved inoculations for the disease, Chopra said.

"We don't know how to eradicate an infection in a mammalian population that's spread by a vector like fleas," Swartzberg said, "so I don't see us being able to eradicate this at this time in our history."

But the plague isn't a top priority for infectious-disease experts or public health officials because it's under control.

"Elimination means the infection is gone in a geographic area and there's no localized transmission," Swartzberg added, "so we've pretty much eliminated the plague as a serious problem."

So, how much should we worry? If we get treatment in a timely manner, there's little to fear, according to experts.

The plague can be treated with antibiotics, Chopra said, but if left untreated it can be fatal.

"If the infection is overwhelming," he said, "then the bacteria can spread fairly quickly into the bloodstream, and then it becomes very dangerous."