Bacterial folliculitis develops when bacteria enters the body through a cut, scrape, surgical incision, or multiplies in the skin near a hair follicle. The bacteria can get trapped and the infection may spread from the hair follicles to the other parts of the body.

Bacterial folliculitis may be superficial or deep. Superficial folliculitis, also called impetigo, consists of pustules which are small-circumscribed elevations of the skin containing pus. The pustules are often surrounded by a ring of redness. Deep folliculitis results when the infection goes deeper and involves more follicles to produce furuncles and carbuncles. These are more serious than folliculitis and can cause permanent damage and scarring to the skin.
Bacterial folliculitis usually occurs in children and adults. Staphylococcus aureus is the most common of bacterial folliculitis causes. It also causes sycosis, a deep chronic infection that involves the entire hair follicle.

Besides the species of streptococcus, pseudomonas, proteus and coliform bacteria have also been indicated as of bacterial folliculitis causes. “Hot Tub” Folliculitis is a condition caused by the pathogen pseudomonas aeruginosa. This disease is often caused due to unsanitary conditions at a spa. The pathogens identified in Gram-negative folliculitis include Klebsiella, Enterobacter, and Proteus species. This type of folliculitis sometimes develops in people receiving long-term antibiotic treatment for acne.

Some superficial follicle infections spontaneously resolve themselves. However, bacterial infections like impetigo, furuncles, carbuncles and “hot tub” folliculitis may not resolve spontaneously and generally require prescription therapy. All these infections are typically diagnosed by clinical presentation, after which predisposing factors are identified and eliminated.

Fungal folliculitis

As the name suggests fungal folliculitis is caused due to fungal infections. Superficial fungal infections are found in the top layers of the skin; deep fungal infections invade deeper layers of the skin. The infection from hair follicles can also spread to blood or internal organs.

The dermatophytic fungus, pityrosporum fungus and the yeast candida folliculitis are the prominent among the fungal folliculitis causes.

Dermatophytic folliculitis is caused most often by a zoophilic species, i.e. fungal species that show attraction to or affinity for animals. The condition presents as follicular pustules around a hardened erythematous (reddened) plaque. A deep fungal penetration causes a high degree of inflammation and determines the extent of hair shaft loss that occurs due to the infection.

Tinea capitis or ringworm of the head is the most important form of pediatric dermatophytic folliculitis. The clinical features of tinea capitis vary considerably depending on the species responsible for the infection. Typically, there is partial alopecia with a varying amount of inflammation.

In the non-inflammatory variants, asymmetrical lesions with short broken hair, 1 to 3 mm in length, are observed. Slight inflammation with scaling may be observed on careful inspection.

The most severe inflammatory reactions are called kerion and produce painful boggy masses studded with pustules. These lesions can result in severe hair loss and significant scarring when the disease is in advanced stages. The diagnosis of tinea capitis is established by identifying the organism in infected hairs under the microscope. A diagnosis is often confirmed by cultures.

Tinea barbae is a superficial dermatophytic infection that is limited to the bearded areas of the face and neck and occurs almost exclusively in older adolescent and adult males. The clinical presentation of tinea barbae includes deep folliculitis, red inflammatory papules and pustules with exudation, crusting and associated hair shaft loss. The two main species causing the infection are T. mentagrophytes and T. verrucosum.

Pityrosporum folliculitis is caused by pityrosporum yeasts resulting in an itchy eruption. The lesions are reddish follicular papules and pustules located mainly on the upper back, shoulders and chest.
Candida folliculitis is caused by the Candida species, ubiquitous fungi that most commonly affect humans.

Viral folliculitis

Viral folliculitis involves a variety of viral infections of the hair follicle. Infection by the herpes simple virus (HSV) often progress to form pustular or ulcerated lesions, and eventually a crust. Infection caused by molluscum contagiosum indicates an immuneosuppressed state which manifests as multiple whitish, itchy papules over the beard area. There are also some reports of folliculitis caused by herpes zoster infection.

Parasitic folliculitis

Parasites causing folliculitis are usually small pathogens that burrow into the hair follicle to live there or lay their eggs. Mites such as demodex folliculorum and demodex brevis are natural hosts of the human pilo-sebaceous follicle.