A fungal infection happens when a fungus grows on or inside your body and causes damage to your tissues. Most of the thousands of fungal species in the environment are harmless, but a few hundred can make people sick, ranging from a mild skin rash to a life-threatening bloodstream infection. Globally, serious fungal diseases affect over 6.5 million people each year and are directly responsible for roughly 2.5 million deaths, a toll that rivals tuberculosis and malaria yet receives far less attention.
How Fungi Infect the Body
Fungi typically enter your body through two routes: direct contact with the skin or inhalation of microscopic spores. A scratch, cut, or even a soggy pair of shoes can let fungi colonize skin and nails. Inhaled spores settle in the lungs and can trigger a pulmonary infection, sometimes spreading further if your immune system doesn’t contain them quickly.
What makes certain fungi dangerous is their ability to shapeshift. Candida, the most common cause of internal fungal infections, normally exists as simple round yeast cells. When conditions favor invasion, it switches into a filamentous form with thread-like structures called hyphae that can physically bore into tissue. This filamentous form is harder for your immune cells to engulf and destroy, and it can even rupture the immune cells that try. Other fungi use different tricks. The species behind Valley fever, for example, transforms into large, thick-walled spheres inside your lungs that resist your body’s defenses and release hundreds of new infectious particles when they burst.
Types of Fungal Infection by Depth
Fungal infections are classified by how deep into the body they reach. Understanding this spectrum helps explain why some infections are a minor nuisance and others are medical emergencies.
Superficial and Cutaneous Infections
These are by far the most common. Superficial infections affect only the outermost layer of skin or hair shafts. You might notice discolored patches on the chest or back (tinea versicolor) or small nodules along a hair strand (piedra). They rarely cause pain or serious health problems.
Cutaneous infections go slightly deeper, affecting skin, hair, and nails. This category includes the “ringworm” family: athlete’s foot, jock itch, scalp ringworm, and fungal nail infections. They’re caused by a group of fungi called dermatophytes that feed on keratin, the protein that makes up your outer skin and nails. Candida can also cause cutaneous infections, particularly in warm, moist skin folds. Symptoms typically include itching, redness, flaking, cracking skin, or thickened discolored nails.
Subcutaneous Infections
These develop in the tissue beneath the skin, usually after a thorn prick, splinter, or other puncture wound pushes fungi into deeper tissue. Sporotrichosis, sometimes called “rose gardener’s disease,” is a classic example. It starts as a small bump at the wound site and can spread along the lymph channels of the arm or leg, forming a chain of nodules. Subcutaneous infections tend to progress slowly over weeks or months and often need prescription antifungal treatment rather than topical creams.
Deep (Systemic) Infections
Deep infections reach the lungs, bloodstream, brain, or other internal organs. Some are caused by fungi that can infect otherwise healthy people. Histoplasmosis, blastomycosis, coccidioidomycosis (Valley fever), and paracoccidioidomycosis all fall into this group. You pick them up by breathing in spores from contaminated soil, and most healthy people either fight them off or experience mild flu-like symptoms. In a minority of cases, especially in people with weakened immunity, the infection can spread throughout the body.
Opportunistic deep infections are a different story. Fungi like Aspergillus, Cryptococcus, and Candida rarely threaten a healthy immune system but can be devastating when defenses are down. Invasive aspergillosis alone affects over two million people a year, with a mortality rate around 85%. Candida bloodstream infections strike about 1.5 million people annually, killing roughly 64% of them. Cryptococcal meningitis, a fungal infection of the membranes surrounding the brain, kills about 76% of the 194,000 people it affects each year, most of them living with advanced HIV.
Who Is Most at Risk
A healthy immune system keeps most fungal exposures from becoming infections. Risk rises sharply when that defense is compromised. The groups most vulnerable include people with HIV/AIDS, organ transplant recipients on immune-suppressing drugs, cancer patients undergoing chemotherapy, people with uncontrolled diabetes, and anyone on long-term corticosteroid therapy.
Broad-spectrum antibiotics are another underappreciated risk factor. Your gut harbors bacteria that actively help train and maintain immune cells specializing in fungal defense. Antibiotics wipe out many of these beneficial bacteria, and the downstream effect is significant. Research shows that antibiotic exposure reduces specific immune cells in the gut that produce key antifungal signals, particularly a protein called IL-17A that coordinates your body’s response to Candida and other fungi. Losing those protective bacteria essentially disarms part of the immune system, which is why vaginal yeast infections and oral thrush so often follow a course of antibiotics.
Common Symptoms to Recognize
Symptoms depend entirely on where the infection takes hold. Skin infections show visible changes: red, scaly, ring-shaped patches for ringworm; white or discolored patches on the torso for tinea versicolor; cracked, peeling skin between the toes for athlete’s foot. Nail infections cause thickening, yellowing, and crumbling of the nail, sometimes with pain as the nail lifts from the bed. Vaginal yeast infections produce itching, burning, and a thick white discharge. Oral thrush appears as white patches on the tongue and inner cheeks.
Systemic infections look very different. A lung infection might feel like a persistent cough with fever, chest pain, and shortness of breath that doesn’t respond to standard antibiotics. If fungi reach the bloodstream, symptoms can include high fever, chills, rapid heart rate, and dangerously low blood pressure. Cryptococcal meningitis causes severe headache, neck stiffness, sensitivity to light, and confusion. Because these symptoms overlap with bacterial infections, fungal causes are often identified late, which partly explains the high mortality rates.
How Fungal Infections Are Diagnosed
For skin and nail infections, a quick in-office test often provides the answer. Your doctor scrapes a small sample of skin or nail, places it on a glass slide with a potassium hydroxide (KOH) solution, and examines it under a microscope. The KOH dissolves skin cells but leaves fungal structures intact, making thread-like hyphae or budding yeast visible within minutes. It’s fast and inexpensive, though not perfect. A fungal culture, where the sample is grown in a lab over days or weeks, remains the gold standard for confirming the species involved. For deeper infections, blood tests, imaging, and sometimes tissue biopsy are needed.
How Fungal Infections Are Treated
Treatment depends on the type and severity. Mild skin infections often respond to over-the-counter antifungal creams, powders, or sprays applied for two to four weeks. Stubborn or widespread skin infections and nearly all nail infections require oral antifungal medication, typically taken for weeks to months depending on the location. Nail infections are notoriously slow to resolve because the nail itself grows slowly; it can take six months for a fingernail and a year for a toenail to fully clear.
All prescription antifungals work by exploiting a key difference between fungal cells and human cells. Fungal cell membranes contain a molecule called ergosterol (human cells use cholesterol instead). One class of antifungals blocks the production of ergosterol, weakening the fungal membrane until the cell can no longer survive. Another class binds directly to ergosterol and punches holes in the membrane, killing the fungus rapidly but sometimes causing more side effects. A third class targets the fungal cell wall, a structure human cells don’t have at all, making it well tolerated.
For systemic infections, treatment is more aggressive. You may receive antifungals intravenously in a hospital setting, sometimes for weeks, followed by months of oral medication. Even with treatment, outcomes for deep fungal infections in immunocompromised patients remain sobering, which is why prevention and early detection matter so much.
Candida Auris: A Growing Concern
One fungal pathogen has drawn increasing alarm from public health agencies. Candida auris is a yeast that was first identified in 2009 and has since spread to healthcare facilities on every inhabited continent. What makes it dangerous is a combination of traits: it resists multiple classes of antifungal drugs, it survives on surfaces for weeks, and it spreads easily between patients in hospitals and nursing homes. The CDC reported 6,304 new clinical cases in the United States in 2024 alone, a number that has been climbing steeply each year. Infections primarily affect people already in healthcare settings with serious underlying conditions, and mortality rates in outbreak reports have ranged from 30% to 60%.
Reducing Your Risk
Most everyday fungal infections are preventable with basic habits. Keep skin clean and dry, especially between the toes and in skin folds. Wear breathable fabrics and change out of sweaty clothes promptly. Use flip-flops in public showers, pools, and locker rooms. Avoid sharing towels, nail clippers, and combs. For vaginal yeast infections, avoiding unnecessary antibiotics and wearing cotton underwear can lower your risk.
For deeper infections, prevention centers on awareness of your environment and your immune status. If you live in or travel to regions where soil fungi are common (the southwestern U.S. for Valley fever, the Ohio and Mississippi River valleys for histoplasmosis), be cautious when disturbing soil, especially during construction or after dust storms. If you’re immunocompromised, your doctor may prescribe preventive antifungals during periods of highest risk, such as after a transplant or during intensive chemotherapy.