Washington faces first outbreak of a deadly fungal infection that's on the rise in the U.S.

Four people in Washington state have tested positive this month for a rare fungal infection that can be deadly.

It's the state's first known outbreak of the fungus, called Candida auris, according to Seattle and King County health officials, though one locally acquired case was detected there in July. No deaths have been reported.

The cluster comes as Candida auris continues to spread in the U.S.: Case numbers have risen every year since 2016.

The pathogen is resistant to some common antifungal medications, and it tends to infect people with weakened immune systems. It's often detected among hospital patients who use catheters, breathing tubes or feeding tubes.

Three of the people who tested positive in Washington were patients at Kindred Hospital Seattle First Hill, a long-term acute care hospital. Their cases were detected through a routine screening program that aims to identify infections before symptoms develop.

A fourth patient had previously received care at Kindred but tested positive at a health care facility in Snohomish County, the public health department for Seattle and King County told NBC News.

The department announced Tuesday that it was notified of the first case on Jan. 10, followed by two more on Jan. 22 and a fourth on Jan. 26. All but one of the patients tested negative for Candida auris when they were admitted to Kindred, suggesting they contracted the fungus sometime afterward.

The health department said one patient developed an infection, which means the fungus entered a part of the body where it would most likely cause symptoms (such as the bloodstream, the ears or an open wound).

The others were colonized with Candida auris — a term used for situations in which people carry the fungus on their bodies and could spread it to others but don’t necessarily get sick from it. People colonized with Candida auris are still at risk of developing infections.

Kindred Hospital Seattle First Hill said in a statement that all three affected patients there are asymptomatic so far and "were appropriately isolated from the rest of the patient population with extra clinical and cleaning precautions to prevent spread."

Candida auris was first identified in Japan in 2009. The Centers for Disease Control and Prevention requested in 2016 that U.S. laboratories report it, and a review of records subsequently identified cases dating to 2013.

Since reporting began, the sharpest increase came from 2020 to 2021, when the number of Candida auris cases rose 94%.

As of December 2022, the fungus had been detected in 36 states. In total, the CDC recorded more than 5,600 Candida auris infections from 2013 to 2022, while more than 13,000 others identified through screenings didn’t have signs of infection.

The CDC attributes Candida auris' rise to increased screening and poor infection control and prevention practices in health care facilities — a challenge most likely made worse by staffing shortages and lengthy patient stays during the Covid pandemic.

"It will reach into every corner of the country for sure," said Dr. Peter Pappas, a professor of medicine at the University of Alabama at Birmingham. "The question is going to be how well can we control it?"

Around 90% of Candida auris strains in the U.S. are resistant to the medication most commonly used to treat other fungal infections. Most strains of Candida auris can be treated with drugs from one of the three main classes of antifungal medicines, but some are resistant to all three.

According to the Seattle and King County health department, Washington's first case was in a Pierce County resident who had been admitted to a hospital there, then transferred to Kindred Hospital Seattle First Hill.

Kindred is the only facility in Washington that screens all new admitted patients for Candida auris — part of a state-led program launched in 2022. The tests involve swabbing the armpits and the groin, which are oily parts of skin that the fungus likes to colonize.

"This is the first time we’ve been able to identify multiple cases in one setting thanks to the screening program. But it’s difficult to say to what extent C. auris is circulating," said Sharon Bogan, a spokesperson for the Seattle and King County health department.

Pappas said the four cases could be a sign that C. auris is spreading more widely in the Seattle area.

"By the time it shows up in a hospital, you know it’s been in the community for quite some time," he said. "It’s just unclear how long that would be."

Pappas added that most of the time, it's easy to distinguish an infection from colonization, though "there are gray areas."

"Those with invasive infections have infections that just have to be treated. Otherwise, the consequences could be pretty, pretty dramatic," he said.

The mortality rate of a severe Candida auris infection in the U.S. is 30% to 60%, but healthy people usually don’t get infected.

Dr. Arturo Casadevall, the chair of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health, said people should actually be reassured when they hear of new cases picked up by active surveillance in their states.

"In a way, it’s disheartening to know that it’s there. But can you imagine if you didn’t have surveillance?" he said.

He added that "most people shouldn’t have to worry about getting it just because it’s in the community."

People can get Candida auris through direct contact with a person carrying the fungus or by touching surfaces or objects that have been contaminated with it. Studies have shown that the fungus can live on surfaces for at least two weeks.

"Once this stuff gets into a hospital, it’s kind of hard to get it out," Casadevall said.

CLARIFICATION (Feb. 2, 2024, 2:35 p.m. ET): This article has been updated to indicate that one of the patients left Kindred Hospital before testing positive for Candida auris. A statement from the hospital has been added, as well.

This article was originally published on NBCNews.com