A fungal infection happens when fungi, organisms that live naturally in soil, air, water, and even on your body, grow out of control and invade your tissues. Most of the roughly 6.55 million people affected by serious fungal infections each year never expected it, because fungi are everywhere and usually harmless. The trouble starts when they find an opportunity: a break in your skin, a warm moist environment, or an immune system that can’t keep them in check.
Why Fungi Can Infect Humans
Fungi are not bacteria. They’re larger, more complex organisms with a rigid outer wall made of a tough material called chitin, the same substance found in insect shells. This wall, layered with sugars and proteins, protects the fungus and makes it harder for your immune system to destroy. Fungi also have cell membranes built around a fat called ergosterol, while your own cells use cholesterol instead. That difference is important: it’s the reason antifungal medications can target fungal cells without damaging yours.
Many disease-causing fungi have another trick. They can switch between two growth forms: a round, compact yeast shape and a branching, thread-like shape called hyphae. This shift lets them adapt to different environments inside your body. In yeast form, they travel easily through your bloodstream. In hyphal form, they can burrow into tissue, anchoring themselves and spreading deeper.
The Most Common Types
Fungal infections fall into three broad categories based on how deep they go.
Skin, Nail, and Mucous Membrane Infections
These superficial infections are by far the most common. Athlete’s foot causes scaling, redness, and itching between the toes or across the sole. Over time, the skin on the soles can thicken, crack painfully, or develop fluid-filled blisters. Ringworm, despite its name, has nothing to do with worms. It produces ring-shaped patches on the body with a slightly raised, scaly border and mild itching that can come and go. Nail fungus thickens, discolors, and distorts the nail over months or years. Yeast infections of the mouth (thrush) and vagina also belong to this group.
All of these are caused by fungi that feed on keratin, the protein in your skin, hair, and nails. They thrive in warm, damp conditions, which is why athlete’s foot tends to develop inside sweaty shoes and why skin folds are common trouble spots.
Subcutaneous Infections
These develop under the skin, usually after a fungus enters through a cut or puncture wound. A scratch from a thorn or a splinter contaminated with soil is the classic route. They cause slow-growing rashes, ulcers, or lumps that don’t heal with normal wound care. People who work with plants, soil, or decaying wood are most at risk.
Deep and Systemic Infections
When fungi reach your lungs, blood, brain, or other internal organs, the infection becomes life-threatening. Globally, invasive fungal infections kill more than 2.55 million people each year, a number that has nearly doubled in recent years. Many of these infections begin when you inhale fungal spores from the environment. Certain fungi live in specific regions: one type thrives in the dry soils of the American Southwest, another in areas with heavy bird or bat droppings across the Midwest and Central United States.
Deep infections are overwhelmingly a problem for people with weakened immune systems. The major risk factors include being in an intensive care unit for a long period, undergoing chemotherapy or organ transplantation, having uncontrolled diabetes or kidney failure, or using broad-spectrum antibiotics for extended stretches. People who inject drugs also face higher risk of fungal bloodstream and heart valve infections. For otherwise healthy people, the immune system typically contains inhaled spores before they cause serious illness.
How Doctors Identify a Fungal Infection
Superficial infections are often diagnosed by appearance alone. Your doctor may scrape a small sample of skin or nail and examine it under a microscope to confirm fungi are present. For deeper or unclear cases, a fungal culture remains the gold standard. A sample of tissue, blood, or fluid is placed in a growth medium and monitored, sometimes for weeks, because fungi grow slowly compared to bacteria. The advantage of a culture is that it identifies the exact species and reveals which medications it’s susceptible to.
Blood tests can also detect markers released by certain fungi into the bloodstream, which helps diagnose invasive infections faster than waiting for a culture to grow.
Treatment Options
Antifungal medications work by exploiting the structural differences between fungal and human cells. The main classes take three different approaches. One group prevents fungi from building ergosterol, the essential fat in their cell membranes, which stops them from growing. Another group punches holes directly in the fungal membrane, destroying the cell outright. A newer class attacks the chitin-rich cell wall itself, causing it to weaken and collapse.
For superficial infections like athlete’s foot or ringworm, over-the-counter antifungal creams, sprays, or powders applied to the skin are usually enough. Treatment typically takes two to four weeks of consistent use, even after symptoms improve. Nail fungus is more stubborn because the medication needs to reach fungus embedded deep in the nail bed. Oral antifungal pills taken for several months are often necessary, and even then, the nail may take six to twelve months to grow out and look normal.
Invasive infections require stronger medications delivered intravenously in a hospital setting. Treatment can last weeks to months depending on the species involved, how far the infection has spread, and the state of the patient’s immune system. Recovery often depends as much on restoring immune function as on the medication itself.
A Growing Resistance Problem
One fungal species in particular has alarmed public health agencies worldwide. Candida auris, reported in more than 40 countries, is highly transmissible in healthcare settings and resistant to multiple classes of antifungal drugs. Some strains resist all three major drug classes, leaving very few treatment options. The species appears to have emerged independently on several continents at roughly the same time, and international travel through healthcare systems continues to introduce it to new regions. Although it primarily threatens hospitalized patients with serious underlying conditions, its rapid spread and resistance patterns have made it a priority surveillance target for the CDC.
Reducing Your Risk
Preventing superficial infections comes down to keeping skin dry and avoiding prolonged contact with warm, moist environments. Dry your feet thoroughly after showering, rotate shoes so they air out between wearings, and wear breathable fabrics in hot weather. In shared spaces like gym locker rooms or pool decks, wear sandals.
Preventing deeper fungal infections is largely about managing environmental exposure. If you work in construction, agriculture, or any job that stirs up soil or dust, reducing airborne particles is key. Wetting soil before digging, using equipment with filtered cabs, and stopping work during dust storms all lower your exposure. Preventing the buildup of bird or bat droppings in buildings eliminates a major source of one common soil fungus. A properly fitted respirator offers protection during high-risk activities like demolition or excavation in contaminated areas.
People with weakened immune systems should be especially cautious about activities that disturb soil, compost, or decaying organic matter. Even routine gardening can introduce spores through small skin breaks or inhalation.