Rosacea is not an infectious disease, and there is no evidence that it can be spread by direct contact with the skin of an affected patient or through inhaling airborne bacteria. However, there has long been a theory that parasites within the sebaceous follicles of the face can stimulate inflammation.
One such organism is the Demodex folliculorum mite, which studies have shown to be more prevalent and active in rosacea patients than in control groups. The mite is 0.3-mm long and is a part of the normal flora of most adults, but becomes a pathogen after it multiplies and invades the dermis, creating an inflammatory response. Aside from rosacea, it has been implicated in folliculitis, ((Infection of one or more of the pockets from which hair grows (follicles))) dyschromias, ((Dyschromia refers to an alteration of the color of the skin or nails. “Hyperchromia” can refer to hyperpigmentation, and “hypochromia” can refer to hypopigmentation. “Dyschromatoses” involve both hyperpigmented and hypopigmented macules.)) pityriasis folliculorum, ((rough, dry and scaly skin)) and inflammatory blepharitis ((Blepharitis
Blepharitis is swelling or inflammation of the eyelids, usually where the eyelash hair follicles are located.)).
Although there is not a standard rest or study for Demodex folliculorum or its smaller partner D. brevis, ((Demodex brevis is one of the two species of face mite that afflict humans. They are very similar to Demodex folliculorum, with just a few differences. They are usually found in the sebaceous glands of the human body.)) a controlled study revealed 10% of all skin biopsies and 12% of all hair follicles contained Demodex mites. This study also suggested that the prevalence of both species increased with age, but D. brevis had a lower prevalence.