The causes of all three types of EPF remain unknown. Some researchers suggest that the condition is caused by an allergic reaction to dermatophytes (parasitic fungi that cause skin infections) or saprophytic fungi (fungi that live on decaying or decomposing organic matter), such as Pityrosporum ovale.This theory is supported by the positive therapeutic response of some patients to oral therapy with itraconazole (antifungal drug). Also, bacteria similar to Leptotrichia buccalis were found in one biopsy specimen of a patient with HIV-associated EPF. The disease responded to oral metronidazole (antibiotic used to treat bacterial infections).

Some researchers speculate that overgrowth of Malassezia or Demodex (the hair follicle mite) might be involved in the development of EPF.

Another theory is that HIV-associated EPF is an autoimmune disorder that causes eosinophils (type of white blood cells that help fight against disease and infection) to mistakenly attack the sebum (oils produced in the skin).

The symptoms for all three variants of the disease are generally the same. About 20% of patients suffer from reddish bumps and/or pimples on the hands or feet, which may be the first signs of the disease.

Patients experience dandruff or dry, scaly skin on the head and trunk of the body. Papulopustules (white-headed pimples that are filled with pus) usually appear around hair follicles on the face and trunk, although the extremities (arms and legs) may also be involved. The classic form tends to affect the palms and soles of the feet. In children, the scalp is more frequently involved. Individual papulopustules may be larger in the classic form (up to 20 to 50mm in diameter) than the HIV-associated or infantile form, which are typically about one to three millimeters in diameter.

The infection may itch or be tender. However, the classic variant of the disease is typically less itchy than the other two forms.

Uncommon symptoms, including nonfolilcular papules (small lesions on the skin that are not near hair follicles) and hives are often seen in infants and HIV patients.

Skin biopsy: A skin biopsy is the standard diagnostic test for eosinophilic pustular folliculitis (EPF). During the procedure, a local anesthetic may be applied to numb the skin that will be sampled. The healthcare provider will then use a scalpel to either cut out or shave off a small sample of skin. The sample is then analyzed under a microscope. Skin biopsies reveal lymphocytic and eosinophilic inflammation around the hair follicles.

Complete blood count: During a complete blood count, a small sample of blood is taken from the patient and analyzed under a microscope in a laboratory. A complete blood count is not considered a diagnostic test for EPF. However, a blood test may detect an increased number of eosinophils in the blood, which may indicate EPF. Eosinophils make up about one to three percent of a healthy person's white blood cells, which is about 350 to 650 eosinophils per microliter of blood.