Ringworm isn’t caused by a worm at all. It’s caused by a group of fungi called dermatophytes, which feed on keratin, the tough protein that makes up your skin, hair, and nails. Three genera of dermatophytes are responsible for virtually all human ringworm infections: Trichophyton, Microsporum, and Epidermophyton. Of these, Trichophyton rubrum is the single most common culprit in North America and northern Europe, followed by Microsporum canis.
The Three Fungal Genera Behind Ringworm
Each of the three dermatophyte genera tends to favor certain body sites, though there’s overlap. Trichophyton species are the most versatile. Trichophyton rubrum is the leading cause of nail infections (onychomycosis) worldwide, while Trichophyton tonsurans is the dominant cause of scalp ringworm in North and South America and the United Kingdom. Microsporum canis is the primary scalp ringworm species in Europe and Asia, and it’s also the species most commonly passed from pets to people. Epidermophyton floccosum, the third genus, is less common and typically causes infections of the groin or feet rather than the scalp.
Geography matters. In parts of Africa, Asia, and Eastern Europe, less common species like Trichophyton violaceum and Trichophyton soudanense are endemic. A newer species called Trichophyton indotineae has been gaining attention for causing body and groin infections that can be harder to treat with standard antifungal medications.
How These Fungi Break Down Your Skin
Dermatophytes have evolved a sophisticated three-step process to eat through the outermost layer of skin. First, the fungi release ammonia, which raises the pH of the surrounding tissue and makes the skin swell. Next comes the critical step: the fungi excrete sulfite, a chemical that breaks apart the disulfide bonds holding keratin’s structure together. This is essentially denaturing the protein, unraveling it like pulling threads from a rope. Finally, fungal enzymes digest the loosened protein into nutrients the fungus can absorb.
This process is why ringworm stays in the outer skin layers rather than invading deeper tissue. Dermatophytes are specialists. They thrive on dead keratin but struggle with the heat, immune defenses, and low-oxygen environment found deeper in the body. Some species grow wide, swollen filaments that bore directly into skin and hair, causing faster, more aggressive infections. Others spread along the surface more slowly.
Where Ringworm Fungi Come From
Dermatophytes reach human skin from three distinct sources, and the source determines which species you’re likely dealing with.
Other people. Species like Trichophyton rubrum and Trichophyton tonsurans spread through direct skin contact or shared items like towels, combs, and gym equipment. Athletes in contact sports and workers who share changing rooms face higher risk. Fungal spores shed onto surfaces can remain viable for weeks under normal room conditions, which is why shared spaces are common transmission points.
Animals. Microsporum canis is the classic example. Cats are the primary reservoir, with some cat populations showing infection rates as high as 100%. In one clinical study, over 83% of animal-transmitted ringworm cases in humans were caused by M. canis. Dogs and farm animals can also carry Trichophyton mentagrophytes and Trichophyton verrucosum. Pets don’t always show visible symptoms, so you can catch ringworm from an animal that looks perfectly healthy.
Soil. A species formerly called Microsporum gypseum (now Nannizzia gypsea) lives in soil and feeds on buried keratin-rich material like hair, feathers, and animal horns. Human infections from soil contact are rare, but when they occur they can trigger unusually intense inflammation, particularly on the scalp in children.
How Location on the Body Changes the Name
Ringworm goes by different clinical names depending on where it appears, even though the same types of fungi are responsible. Tinea corporis is ringworm on the body, producing the classic red, circular, scaly patch. Tinea capitis targets the scalp and is especially common in children; in some countries, more than 25% of children under 10 have scalp ringworm. Tinea cruris, or jock itch, affects the groin and is most common in adolescent and adult males, particularly those who sweat heavily or wear tight clothing. Tinea pedis is athlete’s foot, and tinea unguium refers to nail infections, which can become chronic and impair mobility in older adults.
The species behind each type varies by region. Scalp infections in the Americas are usually Trichophyton tonsurans, while those in Europe are more often Microsporum canis. Body and groin infections worldwide are most frequently caused by Trichophyton rubrum.
How Quickly Symptoms Appear
After your skin comes in contact with dermatophyte spores, symptoms typically show up within 4 to 14 days. The first sign is usually a small, scaly, slightly itchy patch that gradually expands outward while the center begins to clear, creating the ring-shaped pattern that gives the infection its misleading name. On the scalp, it may start as a small area of scaling or hair loss that can be mistaken for dandruff.
People with weakened immune systems are more susceptible to dermatophyte infections, and their symptoms may look atypical, without the classic ring shape, making diagnosis trickier.
Who Gets Ringworm Most Often
Ringworm is one of the most common infections on the planet. Fungal skin infections affect more than 650 million people globally at any given time, and ringworm accounts for over half of those cases. Children are particularly vulnerable to scalp ringworm, while groin and foot infections are more common in teens and adults. Contact sport athletes, military personnel, and people who share locker rooms or gym equipment face elevated risk simply because of proximity and shared surfaces. Pet owners, especially cat owners, have higher exposure to Microsporum canis.