A fungal infection happens when fungi, organisms that are neither plants nor animals, grow in or on your body in a way that causes harm. Many fungi live naturally on your skin, in your mouth, and in your digestive tract without causing problems. Infections develop when these fungi overgrow or when new fungi enter your body through your skin, lungs, or mucous membranes. Fungal infections range from mild skin irritations that clear up in weeks to life-threatening bloodstream infections that kill an estimated 2.5 million people worldwide each year.
How Fungi Get Into Your Body
Fungi reproduce by releasing spores, tiny reproductive particles that travel through the air, soil, and water. You can pick up a fungal infection in several ways: breathing in spores from the environment (soil, dust, bird droppings), walking barefoot in damp public spaces like locker rooms and pool decks, or through a break in your skin such as a cut or thorn scratch. You can also catch certain skin fungi through direct contact with an infected person or animal.
One of the most common triggers is actually medication. Antibiotics kill bacteria, but they also wipe out the helpful bacteria that keep fungi in check. When that balance is disrupted, fungi already living on your body can multiply unchecked. Hormonal changes during pregnancy or menopause, high blood sugar from diabetes, a diet heavy in refined sugars, and chronic stress can all tip the scales in the same direction.
Surface Infections: Ringworm, Athlete’s Foot, and Yeast
Most fungal infections stay on or near the skin’s surface. A group of fungi called dermatophytes cause the infections commonly known as ringworm, athlete’s foot, jock itch, and fungal nail infections. Despite the name, ringworm has nothing to do with worms. It’s named for the ring-shaped rash it produces.
Symptoms typically show up 4 to 14 days after contact with the fungus, and they look different depending on where on the body the infection takes hold:
- Body (ringworm): A red, itchy, ring-shaped rash with a clear or scaly center. On darker skin, the ring may appear brown, gray, or purple rather than red.
- Feet (athlete’s foot): Red, peeling, itchy skin between the toes, most often between the pinky toe and the one next to it. Severe cases can cause blisters and spread to the sole and heel.
- Groin (jock itch): Itchy, scaly, red patches on the inner thigh folds.
- Scalp: A scaly, itchy bald spot that grows larger without treatment.
- Nails: Thickened, discolored, crumbly nails that may separate from the nail bed.
Yeast infections are caused by a different type of fungus, most often Candida. Candida naturally lives in small amounts on your skin, in your mouth, and in the vagina. When conditions shift, whether from antibiotics, a weakened immune system, oral contraceptives, or uncontrolled diabetes, it can overgrow. A vaginal yeast infection causes itching, discharge, and irritation. In the mouth, the same overgrowth is called thrush and appears as white patches on the tongue or inner cheeks.
Deeper and More Dangerous Infections
Fungal infections that move beneath the skin or into the bloodstream and organs are far less common but far more serious. Subcutaneous infections happen when fungus enters a wound, often through contact with plants or soil. These always produce visible skin changes, like nodules or ulcers around the wound site.
Systemic infections occur when fungi reach the lungs and spread through the bloodstream to the brain, heart, kidneys, or other organs. Aspergillus, a mold found widely in the environment, is one of the most common culprits. In healthy people it rarely causes problems, but in someone with a weakened immune system, invasive aspergillosis can spread rapidly from the lungs to the brain and other organs and can be fatal. More than 2 million people develop invasive aspergillosis each year worldwide, with 1.8 million deaths attributed to it. Candida bloodstream infections account for roughly 626,000 cases annually.
Who Is Most at Risk
Your immune system is the main line of defense against fungal overgrowth, so anything that weakens it raises your risk significantly. Cancer patients undergoing chemotherapy or radiation have reduced white blood cell counts that leave them vulnerable. People living with HIV/AIDS face a similar risk. Organ transplant recipients take anti-rejection medications that deliberately suppress the immune system to protect the new organ, and that suppression opens the door to fungal infections. Stem cell transplant recipients face a double hit: the transplant process destroys and rebuilds the immune system, and they also need anti-rejection drugs.
Long-term hospital stays, chronic lung disease, and extended courses of corticosteroids all independently raise the risk. Even outside of a hospital, something as routine as a round of broad-spectrum antibiotics can create the conditions for a yeast infection.
How Fungal Infections Are Diagnosed
For skin infections, diagnosis is often straightforward. A provider scrapes a small sample from the affected area and places it on a microscope slide with a chemical solution called potassium hydroxide, or KOH. The solution dissolves skin cells but leaves fungal structures intact, making them easy to spot under a microscope. If the results are unclear, a small skin biopsy or a fungal culture (growing the organism in a lab to identify it) may be needed. Deeper infections typically require blood tests, imaging, or tissue samples from the affected organ.
How Antifungal Treatments Work
Fungi are biologically closer to human cells than bacteria are, which makes them harder to target without side effects. Antifungal medications exploit the few key differences between fungal cells and ours. Fungal cell membranes contain a fat molecule called ergosterol that human cells don’t use. One class of antifungals blocks the production of ergosterol, so the fungal membrane can’t form properly and the cell dies. Another class punches holes in the membrane by binding directly to ergosterol, causing the cell’s contents to leak out. A third class attacks the fungal cell wall, a rigid outer layer that human cells lack entirely, by blocking the production of a structural sugar that holds the wall together.
Mild skin infections often respond to topical creams or powders applied for a few weeks. Nail infections are notoriously stubborn and may require months of oral medication because the drug needs to grow out with the nail. Systemic infections in the bloodstream or organs require IV or oral antifungals, sometimes for months, depending on how well the immune system recovers.
Drug-Resistant Fungi: A Growing Concern
Just as bacteria can become resistant to antibiotics, fungi can develop resistance to antifungals. The most alarming example is Candida auris, a yeast the CDC classifies as an urgent public health threat. It spreads easily in healthcare settings, resists multiple drugs, and causes high-mortality bloodstream infections. In the United States, clinical cases jumped from fewer than 100 in 2016 to more than 4,500 in 2023.
Testing of over 8,000 U.S. isolates during 2022 and 2023 found that 95% were resistant to one of the most commonly used antifungals. Resistance to a second drug class rose from 10% in 2022 to 19% in 2023. The one remaining first-line treatment still works against 99% of cases, but pockets of resistance are growing, particularly in the Midwest and Northeast. Fewer than 1% of tested samples were resistant to all available drug classes, but even that small number represents a scenario where treatment options become extremely limited.
Preventing Fungal Infections
Most surface-level fungal infections are preventable with basic hygiene. Keep skin folds dry, wear breathable fabrics, and use sandals in public showers and locker rooms. Change out of sweaty clothes and wet swimsuits promptly. If you’re taking antibiotics, be aware that your normal bacterial balance is temporarily disrupted, making yeast overgrowth more likely.
For people with weakened immune systems, prevention focuses on avoiding environmental exposure. That can mean staying away from construction sites or freshly turned soil where fungal spores concentrate, using air filtration in hospital rooms, and in some cases taking preventive antifungal medication during the highest-risk periods after a transplant or during chemotherapy.