Fungal infections are illnesses caused by fungi that invade the skin, nails, lungs, or bloodstream. They range from mild, itchy rashes that clear up with a cream to life-threatening bloodstream infections that kill an estimated 1.5 million or more people worldwide each year. Most people will deal with at least one superficial fungal infection in their lifetime, while serious internal infections primarily strike people with weakened immune systems.
How Fungi Differ From Bacteria
Fungi are neither bacteria nor viruses. They’re a separate category of organism with a unique cell structure that makes them harder to treat. Bacterial cells are relatively simple, but fungal cells are complex and share many features with human cells. That similarity is exactly why antifungal drugs are trickier to develop than antibiotics: it’s difficult to kill a fungal cell without also harming your own.
Two key features set fungal cells apart. First, their cell membranes rely on a fat molecule called ergosterol instead of the cholesterol found in human cells. Most antifungal medications exploit this difference. Second, fungal cells are surrounded by a rigid wall made of chitin and glucan, structural compounds that human cells lack entirely. These components also trigger your immune system when it encounters a fungal invader, which is why fungal skin infections often produce redness and inflammation.
Common Skin, Nail, and Foot Infections
The fungal infections you’re most likely to encounter are superficial, meaning they live on your skin, hair, or nails rather than inside your body. They’re caused by a group of fungi called dermatophytes that feed on keratin, the protein in your outer skin layers and nails. Despite their varied names, they’re closely related conditions.
- Ringworm (tinea corporis): Not a worm at all. It appears as a red, scaly, circular patch on the skin with a raised active border and clearing in the center, giving it a ring-like shape.
- Athlete’s foot (tinea pedis): Typically starts between the toes with redness, peeling, and maceration (soft, white, soggy skin). An acute flare can produce painful blisters. A chronic form called “moccasin pattern” causes dry, thick, scaly skin across the sole and sides of the foot.
- Jock itch (tinea cruris): Most common in adolescent and young adult males. It affects the groin folds but typically spares the genitals themselves.
- Nail fungus (onychomycosis): Causes thickened, brittle, discolored nails. It’s notoriously slow to resolve because nails grow slowly and the fungus is embedded deep in the nail bed.
These infections spread through direct contact with infected skin or contaminated surfaces like locker room floors, shared towels, and sweaty shoes. Warm, moist environments are where dermatophytes thrive, which is why athlete’s foot is so common among people who wear closed shoes for long hours.
Yeast Infections and Oral Thrush
Candida is a yeast that naturally lives on your skin, in your mouth, and in your digestive and genital tracts. In small numbers it causes no problems. But when conditions shift in its favor, it can overgrow and cause infection. Vaginal yeast infections, oral thrush (white patches in the mouth), and skin fold infections in warm, moist creases are all forms of candidiasis.
Antibiotic use is one of the most common triggers. Antibiotics kill bacteria indiscriminately, wiping out the protective bacteria that normally keep Candida in check. This is why many people develop a yeast infection during or shortly after a course of antibiotics. Other triggers include a weakened immune system, uncontrolled diabetes, pregnancy, and hormonal changes.
Serious Internal Fungal Infections
Invasive fungal infections are a different category entirely. They occur when fungi enter the bloodstream, lungs, brain, or other internal organs. These infections are uncommon in healthy people but can be devastating for those with compromised immune systems. Every year, Candida, Aspergillus, Cryptococcus, and Pneumocystis infect an estimated two million people worldwide, and the majority are immunocompromised or critically ill.
Candida is the most common fungal pathogen in intensive care units and among people who have received organ transplants, particularly abdominal organs like the liver or kidney. When Candida enters the bloodstream, the resulting infection (candidemia) can seed organs throughout the body and carries a high mortality rate.
Aspergillus is a mold found in soil, decaying vegetation, and indoor dust. For healthy lungs, inhaling its spores is harmless. But for people undergoing chemotherapy for blood cancers, organ transplant recipients on immune-suppressing drugs, or those on long-term corticosteroids, invasive aspergillosis can destroy lung tissue and spread to the brain. Aspergillus-associated disease is linked to an estimated 1.3 million or more deaths globally each year.
Cryptococcus primarily threatens people with advanced HIV/AIDS. In the developed world, antiretroviral therapy has made cryptococcal meningitis rare among HIV-positive individuals. In the developing world, where access to antiretrovirals is more limited, it remains a common and highly lethal disease.
How Fungal Infections Are Diagnosed
Superficial infections are often diagnosed by appearance alone, but when a rash looks atypical or doesn’t respond to treatment, lab confirmation matters. The most common rapid test involves scraping a small sample of skin or nail, placing it in a potassium hydroxide (KOH) solution that dissolves human cells, and examining what’s left under a microscope. Fungal structures resist the solution and become visible.
More advanced staining techniques use fluorescent dyes that bind specifically to chitin, the structural compound in fungal cell walls, making fungal elements glow under ultraviolet light. This helps distinguish fungi from bacteria that can produce similar symptoms on the skin.
Culture-based testing, where a sample is placed on a growth medium and incubated, remains the gold standard for identifying the exact species. The drawback is speed: dermatophytes can take three to four weeks to grow enough for a final report, which is why doctors often start treatment based on the initial microscopy results.
How Antifungal Treatments Work
Because fungal cells share so much biology with human cells, antifungals are designed to target the few meaningful differences. There are several distinct classes, and each attacks fungi through a different mechanism.
Azoles are the most widely used class, available as both topical creams and oral medications. They block a key enzyme that fungi need to produce ergosterol. Without ergosterol, the fungal cell membrane becomes leaky and structurally unstable, and toxic intermediate compounds build up inside the cell.
Allylamines, commonly used for skin and nail infections, block an even earlier step in ergosterol production. Their killing power comes less from ergosterol depletion and more from the buildup of a precursor molecule called squalene, which reaches levels that puncture the cell membrane from within.
Polyenes work differently. Rather than blocking ergosterol production, they bind directly to existing ergosterol molecules in the fungal membrane and punch physical holes through it. Vital contents leak out and the cell dies. This class is reserved for serious systemic infections because it can also interact with cholesterol in human cells, causing significant side effects.
Echinocandins target the fungal cell wall itself, inhibiting the production of a structural component called beta-glucan. Without it, the wall weakens, new cells can’t separate properly, and the fungus becomes fragile enough to burst. Because human cells have no cell wall at all, echinocandins tend to cause fewer side effects, but they must be given intravenously, limiting their use to hospital settings.
Drug Resistance Is Growing
Antifungal resistance is an escalating public health concern. The CDC identifies resistant Aspergillus, several Candida species, and certain dermatophytes as significant threats. The most alarming example is Candida auris, a species that emerged globally in recent decades and is often resistant to multiple drug classes. Some C. auris cases have resisted all three major classes of antifungals, leaving extremely limited treatment options. It also spreads readily in healthcare settings, making hospital outbreaks difficult to contain.
Part of the resistance problem comes from agriculture. Azole-class fungicides are widely used on crops, and when naturally occurring Aspergillus in soil is exposed to these chemicals, it can develop resistance. That resistant Aspergillus then infects people who have never taken an antifungal drug, yet their infection is already resistant to first-line treatment.
Reducing Your Risk
For superficial infections, prevention centers on keeping skin dry and limiting exposure to contaminated surfaces. Dry your feet thoroughly after showering, especially between the toes. Wear breathable shoes and moisture-wicking socks. Avoid walking barefoot in public showers, pool decks, and locker rooms. Change out of sweaty workout clothes promptly, and don’t share towels, razors, or nail clippers.
For mold exposure at home, humidity control is the most effective strategy. The EPA recommends using exhaust fans or opening windows in kitchens and bathrooms, running dehumidifiers or air conditioners when needed, and drying any damp surfaces within 24 to 48 hours. If mold appears on hard surfaces, clean it with water and detergent and dry the area completely. Absorbent materials like carpet, ceiling tiles, or upholstered furniture that develop mold growth may need to be replaced entirely, because the fungal threads penetrate deep into porous materials where surface cleaning can’t reach.
For people at higher risk of invasive infections, such as those on chemotherapy or immune-suppressing medications after an organ transplant, doctors may prescribe preventive antifungal medication during the period of greatest vulnerability. Avoiding construction sites, compost piles, and other environments with heavy mold exposure can also reduce the chance of inhaling dangerous quantities of fungal spores.