4 of the most dangerous fungi that can make you sick, according to the WHO — from toxic mold to yeast infections

• The World Health Organization released a list of the most health-threatening fungi.
• The ranking is based on fungal resistance to drugs, deadliness, and available treatments.

For the first time, the World Health Organization has published a list of fungus species that are considered threats to human health, citing a recent rise in drug-resistant fungal infections and a lack of research into new treatments.

Millions of fungi exist in the environment, and not all of them make people sick. Several kinds of mushrooms are edible and even beneficial for humans, but there are a few hundred fungi species known to cause dangerous infections.

Fungal infections are becoming more common around the world, in part due to climate change and global travel, according to the WHO report. During the COVID-19 pandemic, invasive fungi increased the risk of hospital-acquired infections for already sick patients.

People with compromised immune systems and respiratory infections are especially vulnerable to harmful fungi, and the medications used to treat them are limited. There are only four classes of antifungal medicines available at the most well-stocked medical locations, and many healthcare settings are limited to fewer options.

Here's what you need to know about the four species considered the most critical threats.

Aspergillus fumigatus

Many people have encountered potentially dangerous fungi without realizing it. Aspergillus is a common mold species found in and around flower beds, decaying plant matter, and damp spots inside homes.

The Centers for Disease Control and Prevention estimates that more than 75,000 people in the US are hospitalized with fungal infections every year, and that at least 20% of those infections are caused by aspergillus.

Because the species is so widespread, it's hard to avoid inhaling aspergillus mold spores. Most people who inhale the mold don't get sick, but aspergillosis is an especially dangerous infection for immune-compromised people.

Candida albicans

If you've ever had a yeast infection, you've experienced candidiasis — a common fungal infection caused by various species of yeast.

Candida albicans is the most common culprit of yeast infections, according to the CDC. The species normally lives on the skin and inside the mouth, throat, and vagina, but it can cause problems if it grows out of control.

For instance, changes to the vaginal environment — due to hormones, medicines, or poor hygiene — can result in itchiness, pain during sex, and abnormal discharge. People with weakened immune systems also may be more likely to experience recurring yeast infections, and they might need stronger medication to treat them.

However, some fungi have already begun to develop resistance to top-line antifungal drugs.

Candida auris

Hospitals have reported a rise in severe yeast infections due to Candida auris, an aggressive and drug-resistant type of yeast, according to the CDC.

Invasive yeast has a tendency to grow where it doesn't belong, such as in the bloodstream and internal organs. C. auris has been known to spread throughout healthcare settings, causing outbreaks of severe infections.

To make matters worse, the fungus is notoriously difficult to diagnose. Many hospitals and laboratories lack the specific technology needed to identify C. auris, so it may be mistaken for a less threatening yeast.

The fungus is resistant to multiple antifungal drugs commonly used to treat yeast infections, making it much more difficult to treat compared to C. albicans, and all the more important to diagnose correctly.

Cryptococcus neoformans

Like aspergillus, cryptococcus is mainly a threat to people with weakened immune systems. Cryptococcus neoformans can be found in environments all over the world. It's ranked a higher threat compared to C. gattii, a similar species that is mostly limited to tropical and sub-tropical climates.

Infection with C. neoformans usually affects the lungs — because it is inhaled — or it can make its way into the brain and spinal cord. Brain infections due to the fungus are called cryptococcal meningitis, which must be treated with a combination of strong antifungals.