The millions of viewers who kicked back and watched HBO’s hit series The Last of Us are unlikely to think the show is grounded in much reality.

However, the drama’s writer has defended the premise that the world could be brought to its knees by a mere fungal infection—and as it turns out, the World Health Organization is worried about it too.

In October 2022, the WHO released the first-ever list of fungal “priority pathogens,” as well as a series of stark warnings. The body outlined that fungal pathogens are becoming both more common and more resistant to treatment, with health professionals having just four classes of medicine available to combat them.

Diseases can be brought on by a range of fungi found everywhere from the great outdoors to indoor surfaces to inside the human body. These can develop into a raft of health problems including skin infections, lung conditions such as asthma or pneumonia, bloodstream infections, ringworm, meningitis, or strains of tuberculosis.

And although many fungal infections are seen as minor everyday problems—such as athlete’s foot or yeast infections—other invasive forms pose a major risk to immunocompromised patients or those with severe underlying health conditions. Experts estimate that around 2 million people die of fungal infections every year.

‘Emerging from the shadows’

While the world was busy battling COVID, fungal infections began “emerging from the shadows” in more treatment-resistant forms, Dr. Hanan Balkhy, the WHO’s assistant director-general of antimicrobial resistance (AMR), said in an October news release.

As a result, the the organization has begun outlining the case for increasing research and development into these pathogens to establish how dangerous and drug-proof they are.

The WHO emphasized when it published its list of priority fungi that there currently isn’t enough evidence or knowledge of these pathogens to fully understand the “burden” they could have on public health.

However, officials at the U.N. agency said countries could get a head start against potentially dangerous fungal infections by taking steps like strengthening their laboratory capacities and ensuring equitable access to existing treatments.

Why haven’t we heard about this before?

Fungal infections have long been hiding in plain sight and slipping through the cracks of public health bodies, experts told Fortune.

“When we think of fungal infections we think of something a bit weird or embarrassing, something that’s trivial and superficial,” Dr. Neil Stone, a specialist in infectious diseases and University College London Hospitals’ fungal infections lead, said in a phone call. “There’s been a legacy of neglect in looking at these diseases, and it’s taken decades to get people to pay attention.”

As well as masquerading behind a humdrum title, these infections haven’t fallen under the remit of any specific public body to address, added David Denning, a professor of infectious diseases at the University of Manchester.

That’s because these diseases span a vast range of practices and are also often misdiagnosed, he said, noting that the number of people contracting and dying of fungal infections is only increasing. The case numbers are going up because of a range of factors: longer life spans, climate change, travel, and resistance to drugs.

Drug resistance is a key issue, added UCL’s Stone, because the limited arsenal medical professionals do have to fight fungal disease is often overused in both humans and animals.

How quickly will this be a problem?

However, there is some good news. Stone said it’s unlikely the rise in these infections will be anywhere near as “explosive” as the COVID pandemic.

He added that the coronavirus outbreak proved how quickly the medical community can come together to troubleshoot when needed, but noted that developing better diagnosis streams and expanding a “pitiful” repertoire of drugs must be top priorities.

Moves to combat the rise of fungal infections will proceed at varying paces, explained Manchester University’s Denning, who also serves as chief executive of Global Action for Fungal Infections (GAFFI).

“There are things we can do quickly which have a big impact,” he told Fortune. “Rapid diagnostics is one of them—we can teach someone how to use a test really easily, and they only cost about $4 each.”

In the long term, training doctors in the field and linking clinical and laboratory engagement is essential, in order to cut down on misdiagnosis and establish which groups of patients are most susceptible to which disease, he added. From there data can be more easily collected and shared by public health bodies.

What does the public need to do?

Being aware that fungal diseases are more than just athlete’s foot is a good place to start, the experts said.

On top of this, there are a few groups of people who might need to chat to a health professional about their symptoms, added Professor Denning.

“If you’re immunocompromised, maybe you have a bad chest or bad skin, or you’re a woman who suffers with recurring yeast infections, then next time you go to the doctor instead of getting a steroid cream or prescription, ask: ‘Is this a fungal infection?'” he said. “The tests are very similar, they’re just not done as routinely. There just needs to be a general awareness of these diseases.”