Fungi interact with your body in surprisingly varied ways, from harmless skin rashes to life-threatening organ infections. Most people encounter fungi as minor nuisances like athlete’s foot or nail infections, but fungi are also permanent residents of your gut and skin, where they play a role in immune health. When the balance tips, whether from a weakened immune system or an aggressive species, fungi can invade deep tissues, release toxins, and trigger inflammatory responses that damage organs. Globally, serious fungal infections cause an estimated 2.5 million deaths each year.
How Fungi Break Down Your Tissue
Fungi are decomposers by nature, and when they colonize human tissue, they use the same basic strategy they’d use on a fallen log. They release enzymes, specifically proteases and lipases, that break down proteins and fats in your skin, nails, and hair. Proteases chop large protein molecules into smaller pieces the fungus can absorb as food. Lipases do the same thing with fats, splitting them into fatty acids and glycerol. This enzymatic digestion is why fungal skin infections cause peeling, flaking, and the gradual destruction of toenails or hair shafts.
Dermatophytes, the fungi behind ringworm, jock itch, and athlete’s foot, are especially good at digesting keratin, the tough structural protein in your outer skin layer. As they consume keratin and spread outward, they create the characteristic ring-shaped lesion of ringworm: an expanding red border with clearing skin in the center. The degree of visible inflammation varies widely between people, largely depending on how aggressively your immune system reacts to the invasion.
Your Immune System’s Response to Fungi
Your body has a specific detection system for fungal invaders. Immune cells carry surface receptors that recognize signature molecules in fungal cell walls, particularly complex sugars called beta-glucans and mannans. When these receptors lock onto a fungal cell, they trigger an alarm that activates your innate immune system, the fast-acting first line of defense you’re born with.
In your lungs, where you inhale thousands of fungal spores daily, specialized immune cells called alveolar macrophages handle cleanup. They engulf spores and digest them using bursts of reactive oxygen, essentially bleaching the fungal cells from the inside. Neutrophils, another type of immune cell, target the thread-like filaments fungi use to grow into tissue. This system works so efficiently that most healthy people never notice they’ve inhaled anything dangerous at all.
When this defense fails, things escalate. People with weakened immune systems, whether from chemotherapy, organ transplants, HIV, or certain medications, lose the ability to contain fungal spores before they germinate and spread. That’s when surface-level encounters become invasive infections.
What Fungi Do to Your Skin
Superficial fungal infections are by far the most common way fungi affect the human body. Dermatophyte infections (tinea) affect the skin, scalp, groin, and feet. The immune response to these infections is primarily cell-mediated, meaning your body sends specialized white blood cells to the site rather than relying on antibodies. This produces the redness, itching, and scaling most people associate with fungal rashes.
Yeast infections are another common surface interaction. Candida species naturally live on your skin and mucous membranes, but overgrowth causes oral thrush (white patches in the mouth), vaginal yeast infections, and skin fold infections in warm, moist areas. These aren’t invasions by a foreign organism so much as a population explosion of something already living on you, typically triggered by antibiotics, hormonal changes, or immune suppression.
How Fungi Damage Your Lungs
Your lungs are the primary entry point for airborne fungal spores, and they bear the brunt of serious fungal disease. Over 2.1 million people develop invasive aspergillosis each year, a lung infection caused by Aspergillus mold spores that are common in soil, compost, and building dust. In healthy lungs, macrophages destroy these spores within hours. In vulnerable people, the spores germinate into branching filaments that penetrate lung tissue, destroying it and sometimes invading blood vessels.
Chronic pulmonary aspergillosis, a slower-burning form, affects about 1.8 million people annually and gradually creates cavities in the lungs. It carries an 18.5% annual mortality rate. Another lung infection, Pneumocystis pneumonia, affects roughly 505,000 people per year, mostly those with severely compromised immune systems, killing about 42% of them.
Even without active infection, fungal exposure can trigger allergic reactions in the airways. Fungal-sensitized asthma affects approximately 11.5 million people worldwide and may contribute to 46,000 asthma deaths each year. In these cases, it’s not the fungus directly destroying tissue but your immune system’s overreaction to fungal proteins that causes airway swelling and constriction.
When Fungi Reach Your Bloodstream and Brain
Invasive fungal infections occur when fungi enter the bloodstream and spread to internal organs. Candida bloodstream infections affect about 1.57 million people annually, with a staggering 63.6% mortality rate. Once in the blood, fungi can seed infections in the heart valves (endocarditis), bones (osteomyelitis), joints, eyes, and the lining of the abdomen. Invasive candidiasis carries an in-hospital mortality rate of roughly 30%.
The brain is particularly vulnerable. Fungal pathogens can cross the blood-brain barrier and cause meningitis, brain abscesses, and strokes. Cryptococcal meningitis, most common in people with advanced HIV, affects 194,000 people per year and kills nearly 76% of them. Fungal infections of the nervous system commonly present as inflammation around the base of the brain, fluid buildup that increases pressure inside the skull, and masses that mimic tumors. These infections progress more slowly than bacterial meningitis but are harder to treat and more often fatal.
Fungal Toxins and Their Effects
Fungi don’t have to infect you to cause harm. Many molds produce mycotoxins, chemical compounds that damage human cells on contact or when ingested. Aflatoxin, produced by Aspergillus species that grow on grains, nuts, and corn, is one of the most potent natural carcinogens known. Chronic low-level exposure increases liver cancer risk significantly.
At the cellular level, mycotoxins disrupt your body’s basic machinery. Some target ribosomes, the structures inside cells responsible for building proteins, effectively shutting down protein production. Others damage mitochondria, the energy-producing components of your cells. Mycotoxin exposure has also been linked to immune dysregulation, where the immune system becomes either suppressed or overactive. This is distinct from infection: you don’t need living fungi in your body to experience the effects of their chemical output.
Fungi That Help Your Body
Not all fungi in your body are harmful. Your gut contains a community of resident fungi, collectively called the mycobiome, that live alongside bacteria on your intestinal lining. These commensal fungi help maintain intestinal balance and play a role in training your immune system. Your adaptive immune system appears to actively manage these fungal residents, suppressing their harmful capabilities while allowing them to remain as beneficial occupants.
This relationship is delicate. When gut fungi populations shift, whether from antibiotics, diet changes, or illness, the resulting imbalance has been linked to inflammatory bowel conditions and broader immune dysfunction. The mycobiome is far less studied than the bacterial microbiome, but it’s increasingly clear that a healthy fungal community is part of normal gut function.
The Growing Problem of Drug Resistance
Treating serious fungal infections relies on a small number of antifungal drug classes, and resistance is rising. Candida auris, a species that emerged in the last decade and spreads easily in healthcare settings, illustrates the problem. Analysis of over 8,000 clinical isolates in the United States during 2022 and 2023 found that more than 95% were resistant to fluconazole, one of the most commonly prescribed antifungals. Fifteen percent resisted amphotericin B, another frontline drug. Less than 1% resisted all three major antifungal classes, but that number represents a threshold the medical community is watching closely.
With only three main classes of antifungal drugs available and an estimated 6.5 million invasive fungal infections occurring globally each year, the margin for lost treatments is thin. Unlike bacteria, fungi are complex cells similar to human cells, which makes developing drugs that kill fungi without harming the patient far more difficult.