Ringworm is caused by a group of fungi called dermatophytes, not by any kind of worm. These fungi feed on keratin, the tough protein that makes up the outer layer of your skin, as well as your hair and nails. The characteristic red, ring-shaped rash appears when the fungus spreads outward from the point of infection while your immune system triggers inflammation in response.
The Fungi Behind Ringworm
Most human ringworm infections are caused by fungi in the genus Trichophyton, though species from two other groups, Microsporum and Epidermophyton, can also be responsible. These dermatophytes are highly specialized organisms. They produce enzymes called keratinases that break down hard keratin into smaller molecules the fungus can absorb as food. Your outer skin layer, hair, and nails are ideal targets because they’re rich in keratin and have relatively few bacteria competing for the same resources.
The infection follows a predictable sequence. First, fungal spores land on your skin and stick to the surface cells. Then the fungus begins secreting those keratin-digesting enzymes, allowing it to invade the outermost skin layer. As it burrows in, it forms thread-like structures called hyphae that spread through the skin and extract nutrients. Your body’s inflammatory response to this invasion is what creates the raised, reddish border of the “ring.” Symptoms typically appear 4 to 14 days after the skin first contacts the fungus.
Three Ways You Can Pick It Up
Dermatophytes fall into three categories based on where they naturally live, and each represents a different transmission route.
From other people. Human-adapted species spread readily through direct skin-to-skin contact or by sharing personal items like towels, combs, and clothing. These species are the most contagious between people and account for a large share of infections, particularly in crowded settings.
From animals. Animal-adapted species live on the skin and fur of dogs, cats (especially kittens and puppies), and other pets including rats. You can pick up the fungus by petting, grooming, or handling an infected animal. Notably, once you catch an animal-adapted strain, passing it to another person is uncommon and tends to fizzle out after a limited number of transfers.
From soil. A smaller number of species live in the environment, typically in soil where they decompose keratin-containing material like shed skin and hair. Gardening or other direct contact with contaminated soil can occasionally lead to infection, though this is the least common route.
Where Fungal Spores Survive
One reason ringworm spreads so effectively is that dermatophyte spores are remarkably durable on surfaces. Shed skin flakes containing the fungus can remain infectious on floors, furniture, towels, and clothing for months. Research on spore survival in skin scales found that different species last different lengths of time: some lose viability within about six months at room temperature, while others can survive 20 months or longer. This means a contaminated locker room bench, shared wrestling mat, or unwashed towel can remain a source of infection long after the original carrier has moved on.
Hard surfaces can be decontaminated with a product labeled as effective against Trichophyton, or with a diluted bleach solution (roughly one-quarter cup of bleach per gallon of water).
Conditions That Raise Your Risk
Dermatophytes thrive in warm, moist environments. That’s why public showers, locker rooms, and pool decks are classic hotspots for transmission. But the conditions on your own skin matter just as much as the environment around you. Fungi colonize more easily when your skin stays damp from sweat, when tight clothing traps moisture against the body, or when minor skin damage gives spores easier access.
Specific risk factors include:
- Using public showers or locker rooms without protective footwear
- Participating in contact sports like wrestling
- Wearing tight shoes, socks, or clothing that holds moisture against the skin
- Sharing sports gear, towels, or other personal items
- Frequent close contact with animals, particularly young cats and dogs
Groin infections (often called jock itch) are more common in adolescent and adult males, largely because physical activity and tight-fitting clothing create the warm, damp conditions fungi prefer in that area.
An Emerging Resistant Strain
Over the past decade, a new species called Trichophyton indotineae has spread across South Asia and begun appearing in other countries, including the first confirmed cases in the United States in 2023. What makes this strain concerning is its frequent resistance to terbinafine, one of the most widely used antifungal treatments. Most clinical labs using standard culture techniques misidentify it as a related species, which can delay appropriate treatment. The strain was only recently recognized as a distinct species based on genomic analysis, and its spread highlights why persistent or treatment-resistant ringworm infections may warrant further testing.
How Ringworm Is Confirmed
A doctor can often recognize ringworm by its appearance, but when the diagnosis isn’t clear, a simple skin scraping can confirm it. Scales are gently scraped from the edge of the rash (where the fungus is most active), placed on a slide, and treated with a solution that dissolves skin cells while leaving fungal structures intact. Under a microscope, the fungus appears as thin, branching filaments, a bit like tiny tree branches.
If species identification matters, particularly when an infection doesn’t respond to treatment, a culture can be grown on a special medium that changes color from yellow to bright red when dermatophytes are present. Newer DNA-based tests can also identify the exact species quickly, which is especially useful for detecting resistant strains like T. indotineae.
Practical Steps to Avoid Infection
Preventing ringworm comes down to keeping your skin dry, minimizing contact with contaminated surfaces, and being careful around animals. Wear flip-flops or sandals in public showers and locker rooms. Wash your hands after handling pets, especially if they have any bald patches or crusty skin. Don’t share towels, socks, combs, or sports equipment. Change socks and underwear daily, and keep your fingernails short and clean since fungi can lodge underneath them.
If someone in your household has ringworm, avoid skin-to-skin contact with the affected area until treatment is underway, and wash shared linens and towels in hot water. Given that spores can linger on surfaces for many months, cleaning high-touch areas with an appropriate disinfectant is worth the effort.