How Fungi Affect Humans: From Disease to Health

Fungi cause over a billion infections in humans every year, yet they also give us cholesterol-lowering drugs, probiotic supplements, and some of the most nutritious foods on the planet. The kingdom Fungi sits in a uniquely ambiguous position relative to human health: a handful of species can kill, many more live harmlessly on and inside us, and a growing number are being harnessed for medicine, nutrition, and environmental cleanup. That range, from lethal pathogen to quiet ally, makes fungi one of the most complex biological forces shaping human well-being.

The Fungal Community Living Inside You

Most people know that bacteria populate the gut, but fungi live there too. This fungal community, called the mycobiome, has only recently attracted serious scientific attention. A 2024 catalog of cultivated gut fungi drawn from healthy volunteers identified 760 fungal genomes spanning 206 species across 48 families, including 69 species that had never been described before.¹ That is a lot of largely uncharted fungal diversity sitting in the intestines of apparently healthy people.

When the mycobiome falls out of balance, however, the consequences can be serious. Studies of people with inflammatory bowel disease have found distinct fungal disruptions alongside the better-known bacterial changes, with altered biodiversity and shifts in fungal composition that suggest fungi play a role in driving inflammation, not just tagging along for the ride.² In Crohn’s disease specifically, inflamed gut tissue shows an expansion of species like Candida, with fungal richness and diversity significantly higher in inflamed mucosa compared with non-inflamed tissue. That increase in fungal diversity tracks with markers of inflammation.³ The relationship between fungi and gut disease is still being untangled, but the emerging picture is that we host a complex fungal ecosystem and keeping it balanced matters more than previously thought.

Skin, Hair, and Nails: Dermatophyte Infections

The most common way fungi cause disease is also the most visible. Dermatophytes, the group of fungi responsible for athlete’s foot, ringworm, jock itch, and fungal nail infections, are specialists in keratin, the tough structural protein that makes up your outer skin, hair, and nails. These organisms produce a suite of enzymes that systematically break down keratin, first snipping the chemical bonds that hold keratin fibers together, then digesting the fragments for food.⁴ Your skin is essentially a meal.

The reason dermatophyte infections are so persistent and prone to recurrence is partly structural. These fungi have evolved multiple ways to gain traction on human tissue, including adhesins that help them stick, pumps that excrete sulfite to soften keratin, and an arsenal of protein-degrading enzymes that let them burrow deeper into the skin.⁵ These infections are usually more annoying than dangerous, but in people with weakened immune systems they can become much harder to treat and occasionally spread beyond the skin.

When Fungal Infections Turn Deadly

Invasive fungal infections occupy a different threat level entirely. When fungi get into the bloodstream, lungs, or brain, the results can be catastrophic, particularly for people whose immune systems are compromised by conditions like HIV, organ transplants, cancer chemotherapy, or long-term steroid use. These opportunistic infections drive high rates of severe illness and death.⁶

One species attracting particular alarm is Candida auris, a multidrug-resistant yeast that emerged in the 2000s and has since spread worldwide in hospital settings. Recent research has uncovered one reason C. auris is so dangerous: a specific enzyme helps it remodel its outer cell wall, making it harder for the immune system to detect and harder for antifungal drugs to kill. When researchers knocked out the gene for this enzyme in laboratory experiments, the fungus became far more vulnerable to immune cells and significantly less virulent in animal models.⁷ Understanding these molecular tricks is critical because the toolkit for treating invasive fungal infections is already small, and resistance keeps narrowing it further.

Some fungal diseases do not require a weakened immune system at all. Valley fever, caused by Coccidioides, is an endemic illness in the southwestern United States. You can pick it up simply by breathing in spores disturbed from dry soil. Field measurements in California found Coccidioides in soil and dust samples around military bases in the Mojave Desert and San Joaquin Valley, posing a real exposure risk for anyone in those areas.⁸ In most people, valley fever causes flu-like symptoms that resolve on their own, but in a fraction of cases it can spread to bones, joints, or the brain.

How Your Body Fights Back

Your immune system has dedicated machinery for recognizing and destroying fungi. A receptor on immune cells called Dectin-1 detects beta-glucan, a sugar molecule that sits on the surface of most fungal cell walls. In mouse experiments, animals lacking Dectin-1 became highly susceptible to Candida albicans infection, confirming that this receptor is not a backup system but a frontline requirement for antifungal defense.⁹ Dectin-1 does not work alone; it partners with another protein, galectin-3, in a two-protein complex that ramps up the inflammatory response when it detects a fungal invader.¹⁰

Other receptors handle different parts of the fungal surface. Dectin-2 and Dectin-3 form a paired unit that recognizes alpha-mannans, sugars found on the growing filamentous form of Candida. When these two receptors link up, they bind fungal surfaces more effectively than either one operating solo, triggering a stronger inflammatory cascade.¹¹ The layered design matters: fungi change shape between yeast and filamentous forms, and the immune system uses different receptor combinations to catch them in each state. When any piece of this detection system is missing or suppressed, the door opens for fungal disease.

Mold, Allergens, and Asthma

You do not need a full-blown infection for fungi to harm you. Inhaling fungal spores and fragments is a potent trigger for allergic airway disease. A meta-analysis of over 30 studies found that early mold exposure in the home significantly increases a child’s risk of developing asthma, with case-control studies showing about a 53% higher odds of asthma in mold-exposed children.¹² The effect was consistent across studies, and although cohort studies showed a somewhat smaller effect, the direction was the same.

One mold particularly tied to severe asthma is Alternaria alternata, a common outdoor fungus found on decaying plant material. Alternaria’s major allergen has protease activity, meaning it can physically cut and activate receptors on airway cells, triggering a rapid calcium surge that kickstarts the allergic inflammation cascade.¹³ Recent immunology research has also identified specific immune-cell population shifts during Alternaria exposure that drive airway hyperresponsiveness, including changes in innate immune cells that do not depend on the classic eosinophil-driven allergic pathway.¹⁴ In other words, mold allergy is not one simple mechanism but several converging pathways, which partly explains why it is hard to treat.

Aflatoxins and the Food Supply

Some fungi are dangerous not because they infect you but because they poison you from a distance. Aspergillus species growing on stored grains, peanuts, and tree nuts produce aflatoxins, a family of potent carcinogens. Aflatoxin B1, the most toxic member, is converted by liver enzymes into a reactive molecule that binds directly to DNA in liver cells. This process creates DNA damage, and one well-characterized outcome is a specific mutation in the p53 tumor-suppressor gene, a genetic change strongly linked to liver cancer.¹⁵ Laboratory work on human liver cell lines has confirmed that the type and frequency of these p53 mutations depend on which specific liver enzymes are present and active, meaning individual variation in liver metabolism can affect how much damage aflatoxin actually does.¹⁶

This is a major public-health issue in developing regions where grain storage conditions are poor and regulatory testing is limited. In wealthier countries, food-safety testing catches most contamination before it reaches consumers, but chronic low-level exposure is still considered a concern worldwide, especially when combined with hepatitis B infection, which multiplies the liver-cancer risk from aflatoxin exposure dramatically.

Medicines That Come From Fungi

For all their capacity to cause harm, fungi have also given us some of the most important drugs in modern medicine. Penicillin, derived from Penicillium mold, is the most famous example, but the list extends well beyond antibiotics. Statins, the cholesterol-lowering drugs taken by hundreds of millions of people, originated from fungal fermentation. Natural statins were first isolated from species including Monascus, Penicillium, Aspergillus terreus, and the oyster mushroom Pleurotus ostreatus. Compactin (mevastatin), the original naturally occurring statin, remains the primary starting material for manufacturing several widely prescribed statin drugs.¹⁷

The newest frontier involves psilocybin, the psychoactive compound found in certain mushrooms. Psilocybin and related psychedelic compounds have shown rapid and lasting therapeutic effects after one or a few doses in clinical trials targeting depression, anxiety, and PTSD. The mechanism appears to involve neuroplasticity: these compounds seem to reopen windows for long-term structural and functional changes in the brain, increasing the nervous system’s sensitivity to environmental input and potentially rewiring maladaptive patterns.¹⁸ Psilocybin specifically stimulates neuroplasticity in the hippocampus and cortex and alters brain connectivity in ways that break down rigid associative networks.¹⁹ Regulatory approval is still underway in most countries, but the research pipeline is moving faster than many expected.

Beyond established drugs, fungi that live inside plants (endophytes) are a rich reservoir of undiscovered bioactive molecules. These organisms produce compounds with anticancer, antibacterial, antiviral, and anti-inflammatory activity, among other effects, largely because they have evolved in competitive environments that selected for chemical creativity.²⁰ Research groups worldwide are screening endophytic fungi from diverse ecosystems for novel drug candidates, making this one of the most active areas in natural-product drug discovery.²¹

Probiotic Yeast and Edible Mushrooms

Not all beneficial fungi arrive in a pill capsule. Saccharomyces boulardii, a non-pathogenic yeast, is one of the best-studied probiotics and has demonstrated effectiveness in preventing and treating several forms of diarrhea, including antibiotic-associated diarrhea and infectious diarrhea. It works through multiple mechanisms: reinforcing the intestinal barrier, modulating the immune response, and actively excluding pathogens from the gut lining.²² Clinical evidence also supports its use in inflammatory bowel disease and irritable bowel syndrome, with its bioactive byproducts, including specialized enzymes, organic acids, and vitamins, helping to calm inflammatory signaling and restore microbial balance.²³

Edible mushrooms offer a different kind of benefit. Mushroom beta-glucans have a distinctive branching structure that is specifically recognized by receptors on immune cells, earning them the status of “biological response modifiers,” meaning they can tune immune activity up or down depending on context.²⁴ Both purified mushroom beta-glucans and crude mushroom extracts have shown immunomodulatory and anti-inflammatory effects in laboratory and animal models, and their potential clinical use has been tested in contexts including recurrent respiratory infections and complications of major surgery.²⁵ The field is still working out exactly how much mushroom consumption translates into measurable immune benefits for the average healthy person, but the underlying biology is sound enough that mushroom-derived beta-glucans are widely used in supplements and functional foods.

Climate Change and Emerging Fungal Threats

Warming temperatures are reshaping the landscape of fungal disease. One influential hypothesis holds that rising global temperatures could increase fungal disease in mammals through two routes: by expanding the geographic range of already-pathogenic fungi into regions they could not previously survive, and by selecting for heat tolerance in environmental fungi that were previously kept in check by the warm body temperatures of mammals.²⁶ The second scenario is the more alarming one because it implies a pipeline of new pathogens coming into existence as the climate warms.

A 2024 review offered a partial counterpoint: given how slowly global temperatures have changed (about 1°C over the last 140 years), it seems unlikely that warming has already driven the evolution of more virulent fungal strains. But the same review agreed that climate change is expanding the geographic range of established pathogenic fungi and suggested that fungal asthma from spore inhalation is a likely side effect of planetary change.²⁷ The “yet” in that paper’s title is doing a lot of work. Range expansion alone is a serious concern when the fungi involved include species that cause fatal lung infections, and the longer warming continues, the harder it becomes to rule out evolutionary adaptation.

Compounding the climate problem is the growing challenge of antifungal drug resistance. Fungi develop resistance through many of the same tricks bacteria use: changing the drug target so the molecule no longer fits, pumping drugs back out of the cell before they can work, and forming biofilms that drugs cannot penetrate.²⁸ With only three major classes of antifungal drugs in clinical use (compared to dozens of antibiotic classes), resistance has far less room to be worked around. Agricultural use of azole fungicides, which share their mechanism of action with clinical antifungal drugs, is suspected of accelerating resistance in the environment. The result is a tightening vise: more fungal disease from range expansion, fewer drugs that work.

Diagnosing Fungal Infections

One major reason invasive fungal infections are so deadly is that they are hard to diagnose quickly. Traditional methods require growing fungi from a patient sample, which can take days or weeks. Newer antigen tests have improved speed dramatically. Lateral-flow devices can detect fungal markers in blood, respiratory fluid, or even urine and return results in about 15 minutes, bringing point-of-care testing into reach for hospitals that previously had to ship samples to specialized labs.²⁹

The diagnostic pipeline is filling with technologies borrowed from other fields. Novel PCR-based assays, microfluidic chips, next-generation DNA sequencing, biosensors, and even artificial-intelligence models are all being developed for fungal identification.³⁰ Faster, more accurate diagnosis does not just improve outcomes for the individual patient; it also reduces the need for broad-spectrum empirical antifungal therapy, which contributes to the resistance problem discussed above.

Fungi Cleaning Up Pollution

Fungi’s ability to break down tough organic molecules is not limited to human keratin or plant cellulose. Mycoremediation, using fungi or their byproducts to clean up environmental pollutants, is a growing field of applied biotechnology. Fungi can degrade heavy metals, agricultural chemicals, pharmaceutical waste, and polycyclic aromatic hydrocarbons, the carcinogenic compounds found in oil spills and industrial runoff.³¹ The approach is cheaper and more environmentally friendly than many conventional remediation methods, and it works on a wide range of contaminants.

Laboratory results can be striking. One strain of Aspergillus fumigatus isolated for remediation research absorbed 98% of zinc and 95% of lead from a mixture of multiple heavy metals and the pesticide lindane, while simultaneously degrading 94% of the lindane within 72 hours.³² The fungus actually used the pesticide as a carbon source, growing more biomass in its presence than without it. Scaling these results from the lab to contaminated field sites remains a challenge, but the underlying capacity of fungi to metabolize chemicals that would persist for decades in soil is genuinely remarkable.

Zombie Ants and Behavioral Manipulation

For a vivid illustration of how sophisticated fungi can be, consider what Ophiocordyceps does to carpenter ants. This parasitic fungus infiltrates an ant’s body, proliferates inside it, and ultimately forces the ant to climb vegetation and bite down on a leaf or twig at a specific height and orientation ideal for spore dispersal. The ant dies locked in that death grip, and a fungal stalk erupts from its head to shower spores on the colony below.

How does a fungus with no nervous system control an animal that has one? The mechanisms appear to involve a cocktail of chemical manipulation. The fungus secretes enterotoxins that may disrupt the ant’s sensory functions, plus small secreted proteins believed to directly affect the brain. Gene expression data show that production of these behavioral compounds, including one related to a tremor-inducing alkaloid, remains high during the critical biting phase just before the fungus is ready to sporulate.³³ The hyperactivity that drives the ant upward may also result from the starvation-like state the spreading fungus creates inside the host. This phenomenon does not directly threaten humans, but it is a powerful reminder that fungi are not passive organisms. They have evolved exquisitely targeted strategies to manipulate other living things for their own reproductive benefit.