Hair fungus is an infection caused by microscopic fungi that invade the hair shaft and surrounding skin, most commonly on the scalp. The medical term for the most widespread type is tinea capitis, often called scalp ringworm (despite having nothing to do with actual worms). It primarily affects children but can occur at any age, and it requires oral medication to clear because topical creams alone can’t penetrate the hair follicle.
What Causes Hair Fungus
The fungi responsible for hair infections belong to a group called dermatophytes, organisms that feed on keratin, the tough protein that makes up hair, skin, and nails. Two main families cause the vast majority of cases: Trichophyton and Microsporum. In the United States, a species called T. tonsurans is now the dominant cause, having replaced older culprits over the past several decades.
These fungi attack the hair in two distinct ways. Some species grow on the outside of the hair shaft, destroying its outer coating and making it extremely fragile. Others burrow inside the shaft itself, leaving the outer layer intact but hollowing out the hair from within. Both patterns lead to breakage, but the distinction matters for diagnosis: hairs infected on the outside often glow greenish-yellow under ultraviolet light, while hairs infected from the inside do not. Since T. tonsurans is an inside-the-shaft invader, the UV light test misses the most common infections in the U.S.
How It Spreads
Hair fungus is contagious and spreads through direct contact with an infected person, animal, or contaminated object. Sharing combs, brushes, hats, towels, pillows, or hair accessories is one of the most common transmission routes. The fungal spores are remarkably resilient and can survive on surfaces for months.
Pets are a significant source. Dogs and cats, especially kittens, can carry ringworm fungi and pass them to humans through handling. The fungus also moves the other direction, from people to animals, and can cycle through a household if both aren’t treated. Children in close-contact settings like daycare, schools, and sports teams face higher risk because of shared equipment and frequent physical contact.
Symptoms to Recognize
The earliest sign is usually one or more round, scaly patches on the scalp where hair appears thin or broken. These patches tend to expand slowly over days to weeks. Up close, the skin looks flaky and silvery, and you may notice tiny black dots scattered across the patch. Those dots are hair stubs that snapped off right at the scalp surface.
Other common signs include:
- Brittle hair that breaks or pulls out with very little force
- Itching or tenderness on the affected areas of the scalp
- Scaly, inflamed patches that gradually increase in size
In some cases, the body mounts an intense inflammatory reaction called a kerion. This appears as a soft, swollen, boggy mass on the scalp that drains pus and develops thick yellow crusting. A kerion is painful and can be mistaken for a bacterial abscess. Hair in and around the area falls out easily. A kerion signals that the immune system is fighting the fungus aggressively, but that reaction itself can cause damage to the hair follicle.
When Hair Loss Becomes Permanent
Most hair fungus infections cause temporary hair loss that resolves once the infection clears. The major exception is kerion. Research on children with kerion found that about 28 percent developed permanent scarring hair loss. The scarring replaces functional hair follicles with fibrous tissue, so hair cannot regrow in those spots. The risk of scarring increases the longer the infection goes untreated, which is why early diagnosis matters so much. Neither the specific type of fungus nor the particular medication used changed the odds of scarring. Speed of treatment was the critical factor.
Less Common Types: Piedra
Not all hair fungus looks like scalp ringworm. Two rarer infections, known as black piedra and white piedra, affect the hair shaft itself without causing the scaly scalp patches typical of tinea capitis.
Black piedra produces small, hard, dark nodules along the hair shaft, primarily on scalp hair. It’s caused by a pigmented fungus and occurs mainly in tropical regions with heavy rainfall. White piedra, by contrast, forms soft, whitish nodules on the hair shaft and can affect not just scalp hair but also beard, armpit, and pubic hair. Both types are chronic, painless infections. They’re most common in tropical climates and relatively unusual in temperate regions.
How It’s Diagnosed
A doctor can often suspect hair fungus based on appearance alone, but confirmation requires additional steps. One quick screening tool is a Wood’s lamp, a handheld ultraviolet light used in a darkened room. Hairs infected by certain Microsporum species glow bright green to yellow-green under this light. The problem is that T. tonsurans, the most common cause in the U.S., does not fluoresce at all. A negative Wood’s lamp result doesn’t rule out infection.
The more reliable test involves plucking a few affected hairs and examining them under a microscope after softening them in a potassium hydroxide solution, which dissolves skin cells and leaves fungal structures visible. A fungal culture, where the sample is grown in a lab over one to two weeks, provides a definitive identification of the species. In cases with a kerion, sampling can be difficult and results sometimes come back falsely negative because the intense inflammation disrupts normal fungal structures.
Treatment Requires Oral Medication
This is the single most important thing to understand about hair fungus: antifungal creams and shampoos alone will not cure it. Because the fungus lives inside the hair shaft and deep within the follicle, only oral antifungal medication taken over several weeks can reach it. Medicated shampoos may be used alongside pills to reduce surface spores and limit spread to others, but they’re supplementary.
Treatment typically lasts six to twelve weeks depending on the medication and the fungal species involved. Your doctor will select the right option based on your age, weight, and the suspected organism. Most people continue treatment for at least two weeks after symptoms visibly resolve to make sure the fungus is fully eliminated. Some infections, particularly those caused by Microsporum species, take longer to clear than others.
During treatment, you can generally continue normal activities including school and work. Hair regrowth in the affected patches usually begins within a few weeks of starting medication, though full regrowth can take several months.
Preventing Spread and Reinfection
Because fungal spores persist on objects and surfaces, hygiene steps are essential both during and after treatment. Don’t share combs, brushes, hats, towels, pillowcases, or hair ties. Wash your hands thoroughly after touching the affected area or applying any topical treatment.
If a pet in the household is suspected as the source, they need veterinary evaluation and treatment simultaneously. Otherwise the fungus will ping-pong between family members and the animal indefinitely. Clean bedding, grooming tools, and furniture that pets frequent.
Keep the affected area clean and dry. In households with children, encourage regular handwashing and avoid head-to-head contact during active infections. Families dealing with recurrent infections should check all household members, including those without symptoms, since some people carry the fungus without showing visible signs and continue to spread it.