These Fungal Pathogens Are the Most Harmful to Human Health

Fungal infections kill more than 1.5 million people each year, and the WHO warns they are becoming more prevalent and resistant to treatment

The fungus Aspergillus fumigatus, as seen under an optical microscope.
The fungus Aspergillus fumigatus, as seen under an optical microscope. It's listed as a critical priority on the WHO's new report of most harmful fungal pathogens.  BSIP / Education Images / Universal Images Group via Getty Images

From molds to yeasts to inhalable spores, 19 fungal pathogens have been identified by the World Health Organization as the biggest threats to human health, according to a report released Tuesday.

Fungal pathogens are historically understudied, leading to gaps in the understanding, surveillance and treatment of infections, writes ReutersJennifer Rigby.

“The bottom line is that invasive fungal infections are becoming more prevalent, but frequently they are not recognized in patients and not correctly treated,” Carmem L. Pessoa-Silva, a WHO official, said at a news conference Tuesday, as reported by the New York TimesAndrew Jacobs. “We do not have a real sense of the size of the problem.”

The rise in fungal infections comes as the pathogens build resistance to antifungal drugs, aided by overuse of antifungals in agriculture, according to Reuters. Additionally, the WHO says climate change is increasing the geographic range of some pathogens and leading to more infections.

The U.N. health agency’s list separates the fungi into three tiers: critical, high and medium priority. It aims to provide policymakers with guidelines for directing resources toward tackling the highest priority fungi, writes NPR’s Max Barnhart. Officials also said they hoped it would create a larger sense of urgency among officials and experts, per the Times.

“This is long overdue given that fungal diseases have long been neglected even as the problem grows at an exponential rate,” Cornelius J. Clancy, an infectious diseases doctor at the VA Pittsburgh Health Care System who did not contribute to the WHO’s report, tells the Times.

All of the fungi on the WHO’s list are microscopic—still, many of them are deadly. Two of the four marked as critical concern—Cryptococcus neoformans and Aspergillus fumigatis—infect the lungs and cause pneumonia-like symptoms that can worsen and be fatal, writes NPR. The other two are pathogenic yeasts that can cause serious or life-threatening infections.

Most fungi aren’t harmful to people, though—a study released earlier this year found that of the over 150,000 known species, only about 200 can infect humans.

But people who are already severely ill with diseases like cancer or tuberculosis, or have other underlying conditions making them immunocompromised, are at an increased risk for fungal infections. People hospitalized with Covid-19 also developed infections at a high rate: One pathogen listed as high priority, called Mucorales, caused an outbreak of mucormycosis during the Covid-19 pandemic in India last year, leading to several surgeries to remove the fungus, writes the Times.

“While an intact human immune system can easily fend off these fungal pathogens, those who are immunocompromised can’t, which might lead to a clinical infection,” Michelle Momany, a fungal researcher at the University of Georgia who did not contribute the WHO report, tells NPR. Public health officials say that healthy people are unlikely to get a severe fungal infection, per the Wall Street Journal’s Dominique Mosbergen.

Still, fungal infections kill at least 1.5 million people annually. And they contribute to the deaths of five million others, per the Times. According to the CDC, more than 75,000 people in the U.S. are hospitalized with fungal infections each year.

“The general public doesn’t appreciate how serious fungal infections can be,” Felipe Santiago-Tirado, a fungal infection expert at the University of Notre Dame, says to the Wall Street Journal. “Mortality can be very high.”

The true death toll from fungal pathogens could be much greater than scientists know, health officials say, because many hospitals and clinics don’t have the tools to test for them, per the Times. As a result, infections might not be identified until an autopsy, NPR reports.

Only four types of treatments for fungal infections exist, and there are very few new possible treatments in development. Officials say they hope the new report will spur investment in research to find better diagnostic tools and treatments, per NPR.