Infectious Agents

In this section, we explore the range of infectious agents that can cause hair loss, an important aspect often overlooked in alopecia discussions. Common conditions like Ringworm, Folliculitis, Piedra (Trichomycosis Nodularis), Demodex folliculorum, and Seborrheic dermatitis not only affect a significant number of individuals but also have a notable impact on hair health. Each of these conditions exhibits distinct characteristics and affects the scalp and hair follicles in unique ways. Gaining an understanding of these infectious agents is essential for accurate diagnosis and effective treatment.


Ringworm has nothing to do with worms; it is actually a fungal infection. Ringworm is primarily an infectious skin condition that can occur anywhere on the body. If it develops on the scalp, it can cause patches of hair loss. When it occurs on the scalp, the professional term for ringworm is “tinea capitis”. Ringworm is the same thing as athlete’s foot, and the same kind of fungal infection can also affect the nails. Ringworm of the scalp usually starts as a small pimple that progressively expands in size, leaving scaly patches of temporary baldness. The fungus gets into the hair fibers in the affected area, and these hairs become brittle and break off easily, leaving a bald patch of skin. The affected areas are often itchy, red, and inflamed, with scaly patches that may blister and ooze. The patches are usually redder around the edges and have a more normal skin tone in the center. This may create the appearance of a ring, hence the name “ringworm.”

Worldwide, the fungus Microsporum audouinii is a very common cause of ringworm. However, Trichophyton tonsurans is increasingly recognized as a cause of tinea capitis, especially in Latin American countries. Other fungi that may cause tinea capitis include Trichophyton schoenleinii, Trichophyton megninii in Southern Europe and Africa, and Trichophyton violaceum in the Middle East. The fungus Microsporum gypseum can sometimes cause tinea capitis. This fungus is common in soil and can be transferred to humans through contact with infected animals. Ringworm can also be contracted from pets that carry the fungus, with cats being particularly common carriers. Ringworm is contagious and can be transmitted through direct skin-to-skin contact or contact with contaminated items such as combs, unwashed clothing, and shower or pool surfaces.

Treatment for ringworm varies depending on the specific fungus involved. Some types of ringworm infections will resolve spontaneously, so no treatment may be necessary. However, most commonly, an antifungal medication called Griseofulvin is used. Griseofulvin is highly effective against fungi in hair and skin, but it is not as effective against yeast or bacterial infections. Griseofulvin gradually accumulates in the skin and hair, binding with keratin, a key component of hair, skin, and nails. This blocks the fungus from infecting the keratin. In recent years, some fungi causing tinea capitis have shown resistance to Griseofulvin, necessitating higher doses and longer courses of treatment. As an alternative, newer antifungal drugs like Terbinafine, Itraconazole, and Fluconazole can be prescribed.


Folliculitis is a term for localized inflammation of hair follicles. It resembles acne, with small rings of inflammation surrounding the opening of a hair follicle. In the early stages of folliculitis, the hair fiber may still be present in the center, but as the condition progresses, the hair often falls out. Severe folliculitis can lead to permanent destruction of hair follicles, resulting in small bald patches. Some forms of folliculitis are non-infectious and caused by oils and greases that clog hair follicles. However, folliculitis is usually due to a bacterial infection, most commonly by Staphylococcus aureus. “Hot tub folliculitis” is caused by Pseudomonas aeruginosa, which thrives in inadequately chlorinated water. Viral, fungal, or yeast-induced folliculitis can also occur, involving agents like Herpes simplex, Herpes zoster, Pityrosporum ovale, and Trichophyton rubrum. Non-prescription topical antibiotics like bacitracin, mycitracin, or neomycin can be used to treat minor folliculitis, while more serious infections may require oral antibiotics such as erythromycin or antifungal treatments like Griseofulvin for fungal infections.


Piedra (Trichomycosis Nodularis) is a condition where hair fibers are infected by a fungus. The visible sign of a piedra infection is the development of hard nodules on hair fibers. “Piedra” is Spanish for stone. These nodules are concretions of hyphae and fruiting bodies of the fungus, known as an ascostroma, from which fungal spores are released. There are two main types of piedra: black piedra and white piedra, named after the color of the nodules formed on the hair fibers. Black piedra is caused by the fungus Piedraia hortae and is mostly found in tropical countries, while white piedra is due to Trichosporon beigelii and is more common in Europe and southern parts of the USA.

The infection can affect hairs on the scalp, body, and genital areas. Usually, the infection is relatively benign. In parts of Malaysia, the nodules of black piedra are considered attractive, and traditionally, women encourage its growth by burying their hair in the soil while sleeping. However, severe infections can weaken hair fibers, leading to patchy or diffuse hair loss. Treatment generally involves shaving off affected areas or applying topical agents like salicylic acid or formaldehyde. White piedra is resistant to azole-based antifungals, but these treatments can be used for black piedra. Oral therapy with ketoconazole or terbinafine has also been effective.

Demodex folliculorum

In some communities, there’s a popular belief that Demodex folliculorum contributes to hair loss and that removing these creatures will promote hair regrowth. Demodex is a tiny worm-like creature that resides on the skin and in hair follicles. It feeds on dead skin and oils, thriving in hair follicles where these substances are abundant. While common, Demodex is mostly benign. We are born free of Demodex, but through contact with others during childhood, our skin can become infected by them. About 70% of adults have some Demodex in their hair follicles. In most cases, their presence goes unnoticed. They can cause irritation, especially in eyelashes, but this is the extent of their impact. They do not cause hair loss.

Seborrheic dermatitis

Seborrheic dermatitis is not an infectious disease, although it can involve infection. It is primarily a skin condition, but it can also lead to temporary hair loss if it occurs on the scalp or other areas with terminal hair. The dermatitis manifests as scaly, sometimes oily, inflamed skin that can be itchy or even painful. This inflammatory condition’s cause is not well understood, but genetics seem to play a role, with Caucasians of Celtic descent being particularly susceptible. The sebaceous glands attached to hair follicles produce an excessively rich form of sebum, likely triggered by androgen steroids. Hormone fluctuations, such as during puberty, can activate seborrheic dermatitis. It can also be observed in newborns when maternal androgens are transferred across the placenta. Conditions like Parkinson’s disease, head injuries, stroke, stress, and chronic fatigue can be associated with seborrheic dermatitis or exacerbate it.

Excessive sebum production in seborrheic dermatitis can lead to the proliferation of skin flora. The yeast Pityrosporon ovale (also known as Malassezia furfur) increases with the intensity of seborrheic dermatitis, causing more irritation and inflammation. Although the inflammation isn’t specifically directed at hair follicles, they can still be adversely affected if in the vicinity of inflammatory cells. Hair follicles find inflamed skin an unhealthy environment for growth, leading to non-specific diffuse hair loss. This hair loss should be reversible with a reduction in inflammation. Although seborrheic dermatitis involves yeast proliferation, it’s important to note that it’s not infectious; it originates from the individual’s own skin. While everyone has various types of yeast on their skin, the issue in seborrheic dermatitis is the overgrowth of these yeasts.

Several treatments are available for seborrheic dermatitis. The simplest involves medicated anti-dandruff shampoos to control skin proliferation and scaling. Different shampoos might be recommended for alternating use, each with its specific activity. These shampoos may contain sulfur, selenium sulfide, zinc pyrithione, tar, salicylic acid, or oil of Cade. Azole-based shampoos like Ketoconazole have also become available over the counter. Dermatologists might also prescribe antibiotics to control skin flora and indirectly reduce inflammation. Direct inflammation can be treated using corticosteroid creams or lotions. Seborrheic dermatitis can be persistent, so consistent treatment and preventative measures are important, even when symptoms improve.