Some people believe Demodex folliculorum contributes to hair loss and that removing it will enable hair regrowth. But the organism does not cause hair loss.
Demodex is a little worm-like creature that likes to live on skin and in hair follicles. It feeds on dead skin and oils, so it particularly likes to live in hair follicles where there are lots of both.
Humans are born free of Demodex, but during childhood, through contact with others, the skin can become infected with it. For the most part, we never know they are there. They are benign, if repulsive, little creatures. The most common problem with Demodex is that they may cause irritation, particularly in the eyelashes. If you have itchy eyelashes, Demodex may be the problem.
However, this is as much as Demodex can do to you. It does not cause hair loss.
Seborrheic dermatitis is first and foremost a skin condition, but it can involve infection and temporary hair loss if the dermatitis is located on the scalp or other skin areas. The dermatitis causes scaly, sometimes oily, inflamed skin that can be itchy or even painful to touch.
This is an inflammatory condition that is not well understood, although there does seem to be a genetic component and Caucasians, particularly of Celtic descent, are most susceptible. Some newborns develop seborrheic dermatitis when maternal androgens are passed from the mother to the baby across the placenta. Conditions such as Parkinson's disease, head injury, and stroke can also be associated with seborrheic dermatitis, and stress and chronic fatigue can make it worse. Times of hormone fluctuation, such as during puberty, can activate the onset.
In part, the trigger for seborrheic dermatitis may be androgen steroids. The sebaceous glands attached to the hair follicles begin to produce a very rich form of sebum. The sebum contains fewer free fatty acids and squalene but increased amounts of triglycerides and cholesterol. The excess, rich sebum production triggers the proliferation of skin flora. Yeast Pityrosporon ovale (also called Malassezia furfur) has been shown to increase in numbers with the intensity of seborrheic dermatitis. This excessive yeast proliferation causes more irritation and inflammation.