Demodex refers to 2 types of microscopic eyelash mites that live on humans:
Demodex folliculorum, which live within hair follicles on eyelashes
Demodex brevis, which live in sebaceous glands (oil glands) in the skin around the eyes
Most of the time, these mites are harmless and will go unnoticed.  However, when they multiply and their population get out of control, Demodex can cause many ocular diseases like conjunctivitis, chalazia, blepharitis and contribute to dry eye symptoms, including eyelid itchiness and falling eyelashes.
These mites can also cause unwanted symptoms and skin problems like rosacea, androgenetic alopecia, non-specific facial dermatitis (acne-like blemishes).  Demodex is transferred between hosts through contact with hair, eyebrows, and the oil glands of the face.