Hair fungus is caused by a group of fungi called dermatophytes that feed on keratin, the protein that makes up your hair and skin. The most common culprit in the United States, Canada, and Central America is a species called Trichophyton tonsurans, which spreads from person to person. The medical name for this infection is tinea capitis, and it primarily affects the scalp, though the fungus can also invade eyebrows and eyelashes.
The Fungi Behind the Infection
Two main groups of fungi cause hair infections: Trichophyton and Microsporum. Which one you’re dealing with depends largely on where you live. In North America, T. tonsurans dominates and has largely replaced older species that were common decades ago. In Europe, Microsporum canis and Microsporum audouinii remain widespread. In Africa and parts of Asia, T. violaceum and M. canis are the most frequently identified species.
These fungi attack the hair shaft in two distinct ways. Some invade from the outside, coating the exterior of the strand with spores. Others burrow inside the hair shaft itself, filling it with spores from within. The inside invaders, like T. tonsurans, are particularly sneaky because they cause hair to become fragile and snap off right at the scalp surface, leaving behind tiny dark dots. The outside invaders tend to break hair slightly above the skin, leaving short visible stubs.
A third, less common pattern produces thick yellowish crusts on the scalp called scutula. This form is typically caused by T. schoenleinii and can lead to permanent hair loss if left untreated.
How Hair Fungus Spreads
The primary route of infection is direct contact with someone who has it. This makes schools, daycares, and households with young children common settings for outbreaks. Children can pass the fungus through casual head-to-head contact during play, sleepovers, or sports.
Contaminated objects are another major pathway. Combs, brushes, hats, pillowcases, towels, and hair accessories can all harbor fungal spores. These spores are remarkably resilient. Without proper cleaning, they can survive on surfaces for extended periods, waiting to infect the next person who uses that item. Sharing grooming tools is one of the most preventable risk factors.
Animals, especially cats and dogs, can carry Microsporum canis and pass it to humans through petting or handling. Interestingly, some animals act as “mechanical carriers,” meaning they pick up spores from a contaminated environment and carry them on their fur without ever developing an infection themselves. You can catch the fungus from an animal that looks perfectly healthy.
Who Gets It and Why
Children between ages 3 and 14 are the most commonly affected group. Several factors make kids more vulnerable: their scalps produce less of the natural fatty acids that help adults resist fungal colonization, they’re more likely to share personal items, and they have more frequent close physical contact with peers.
Warm, humid environments encourage fungal growth, so outbreaks tend to be more common in tropical and subtropical climates. Overcrowded living conditions, poor hygiene practices, and minor scalp injuries (from tight braiding, scratching, or cuts) all increase the risk of infection. The fungus needs a way in, and any small break in the skin gives it an entry point.
Adults can get hair fungus too, though it’s less common. When it does occur in adults, it often affects people with weakened immune systems or those in close contact with infected children.
What Hair Fungus Looks Like
The infection shows up in several distinct patterns, and recognizing them can help you catch it early.
Black dot pattern: Hair shafts break right at the scalp surface, leaving dark spots that look like tiny black dots scattered across a bald patch. This is the hallmark of T. tonsurans infection and the most common presentation in North America. The “dots” are actually the broken ends of infected hair strands still embedded in the follicle.
Gray patch pattern: Hair breaks just above the surface, leaving short stubs visible on an oval or round area of the scalp. The patch often looks dull and grayish because the broken hairs are coated in fungal spores. This pattern is more typical of Microsporum infections.
Kerion: This is the most alarming presentation. It happens when the body mounts a strong inflammatory response against the fungus, producing a large, boggy, swollen mass on the scalp. A kerion is painful, may ooze pus, and develops crusty blisters on its surface. It can be mistaken for a bacterial abscess, but it’s actually a reaction to the fungal infection itself. Without treatment, a kerion can cause permanent scarring and hair loss in the affected area.
Across all types, itching and flaking are common. Some people also develop swollen lymph nodes at the back of the neck as their immune system responds to the infection.
Incubation and Contagion Timeline
After exposure, tinea capitis typically takes 10 to 14 days before symptoms appear. During this window, the fungus is establishing itself in the hair follicles, and the infected person may already be shedding spores without realizing it.
Once treatment begins, contagiousness drops quickly. Public health guidelines generally consider an infected person safe to return to school or group activities the day after starting appropriate antifungal therapy. However, the infection itself takes much longer to fully clear. Treatment courses typically run 4 to 8 weeks because the medication needs time to work through the full hair growth cycle and eliminate fungus deep within the follicle.
Why Topical Creams Alone Don’t Work
Unlike fungal infections on smooth skin (like athlete’s foot or body ringworm), hair fungus lives inside the hair shaft and follicle, where creams and shampoos can’t reach in sufficient concentration. This is why tinea capitis requires oral antifungal medication. The drug enters your bloodstream and reaches the fungus from the inside, depositing into newly growing hair and nail tissue.
Medicated shampoos are often used alongside oral medication, but their role is to reduce the number of spores on the scalp surface and limit spread to others. They won’t cure the infection on their own.
Keeping It From Spreading
Preventing transmission comes down to hygiene practices and awareness. Avoid sharing combs, brushes, hats, helmets, hair ties, towels, and pillowcases with anyone who has or may have the infection. If someone in your household is being treated, give them their own set of grooming tools and linens.
Contaminated items can be disinfected by washing fabrics like towels, pillowcases, and hair covers in hot water (70 to 80°C, or roughly 160 to 175°F) with detergent, then drying them in a clothes dryer on a hot setting. Non-fabric items like combs and brushes can be soaked in a bleach-based disinfectant solution. Hard surfaces that can’t be submerged can be wiped with 70% alcohol and allowed to air dry.
If you have pets, especially cats, and someone in your household develops a scalp fungal infection, it’s worth having the animal checked by a veterinarian. Even if the pet shows no signs of ringworm, it could be carrying spores picked up from the home environment. Treating all potential sources at once is the fastest way to break the cycle of reinfection.