The fungus was detected in more than half of all U.S. states last year.

In a research paper published on Tuesday, fungal disease experts from the U.S. Centers for Disease Control and Prevention (CDC) shared their findings on how the fungus Candida auris had spread in American healthcare facilities between 2019 and 2021.

Describing the disease as “an emerging fungal threat,” government researchers said 17 states had identified their first case of Candida auris during their analysis period.

Candida auris infections reported in healthcare settings across the U.S. rose by more than 200% during the same period, the study found.

Meanwhile, the number of screening cases—where the fungus was detected but was not causing an infection—tripled to 4,041 in healthcare settings between 2020 and 2021.

The increase in cases occurred for many reasons, the CDC said, including poor infection control practices in healthcare facilities.

Candida auris’s spread may have worsened between 2019 and 2021 due to strain on health services during the COVID-19 pandemic, officials noted.

According to CDC surveillance data, cases of Candida auris have been recorded in more than half of U.S. states over the past 12 months, with Nevada, California and Florida reporting the most infections.

Throughout the U.S., 2,377 cases were confirmed in the 12 months to December 2022, with almost 6,000 screening cases recorded across the country.

CDC epidemiologist Dr. Meghan Lyman said in a statement on Monday, ahead of the paper’s publication, that the “rapid rise and geographic spread of cases is concerning.”

What is Candida auris?

Candida auris is a fungal disease that, according to the CDC, “presents a serious global health threat.”

Earlier this year, it was named on the WHO’s first-ever list of fungal “priority pathogens.” Candida auris was named as one of four “critical” threats—the highest risk category—on the list of 19 fungal diseases.

While generally not a threat to healthy people, the infection, caused by a type of yeast, can cause severe illness if it enters the bloodstream and spreads throughout the body.

As well as bloodstream infections, Candida auris can also cause ear and wound infections, and it can sometimes be present in people without causing any symptoms or illness.

This means the fungus can be carried on people’s skin without them knowing it, allowing it to spread to others.

People who have been hospitalized for a long time, have serious medical conditions, and those who have recently been given antibiotics or antifungal medications, are thought to be at higher risk of infection.

Those who have recently spent time in a nursing home and have lines or tubes—such as feeding tubes or central venous catheters—entering their bodies appear to be at the highest risk, according to the CDC.

While Candida auris has caused outbreaks in hospitals and other healthcare settings, the fungus is difficult to identify with standard lab tests, and without specific technology, it can be misidentified.

How does it spread?

Candida auris was first identified in Japan in 2009.

Since then, it has been found in many countries and caused prolonged outbreaks in several nations including Pakistan, Venezuela, Spain, Israel, South Africa and the United States.

The disease can spread through contact with contaminated surfaces or equipment, especially in healthcare settings, but it can also spread between people.  

Health officials say more research is needed to understand how the fungus spreads.

However, infections have mostly occurred in hospital patients who had already been hospitalized for other reasons.

Is it fatal?

It can be.

Statistics from a limited number of patients suggest that between 30% and 60% of people with Candida auris infections have died.

However, many of those who passed away with a Candida auris infection had other serious illnesses that raised their risk of death.

Dr. Neil Stone, a consultant in infectious diseases and microbiology and specialist in fungal infections at University College London Hospital, told Fortune in a phone call on Tuesday that the rapid rise of Candida auris was “a worrying story.”

“The reassuring part is it's generally not harmful to people who are healthy and have a functioning immune system,” he said. “It's what we call an opportunistic pathogen, which means for most people it’s harmless, but for certain patients who have weakened immune systems it can be extremely serious or life-threatening.”

Is there a cure?

According to the CDC, most Candida auris infections are treatable with a specific class of antifungal drugs.

However, Candida auris is often multidrug-resistant, with some strains resistant to all three of the antifungals available for treatment.

Stone told Fortune on Tuesday that the drug resistance had arisen because the threat posed by fungal infections had been “neglected for decades.”

“Compared to other infectious diseases, like bacterial infections and viruses, they’ve been dismissed as something not important or a bit weird or embarrassing, or just causing things like athletes foot—and so there's been very little research into their diagnosis and treatment,” he said.

“So we've been left with only three types of drugs—and pretty much all strains of Candida auris are resistant to one of the three drugs.”

He explained that most strains of the fungus were resistant to the drug class that was the only oral form of antiviral treatment. That left only intravenous treatments for the disease.

“But even worse than that, sometimes it's resistant to two of the three types, and there are even strains which are resistant to all three types,” he said. “It's incredibly drug-resistant—the first fungal superbug.”

 CDC data shows that 90% of infections are resistant to at least one antifungal medicine, while 30% are resistant to at least two.

In cases of multidrug-resistant infections, patients may be treated with multiple different classes of antifungal treatments administered at high doses.

However, Stone warned that a prolonged lack of investment into research on fungal diseases had created an unwelcome predicament.

“The neglect to fungal infection [research], a lack of diagnostics and treatments have led us to a tricky situation where if it gets out of control, we're really poorly equipped to deal with it,” he said.