Demodex folliculorum, or the demodicid, is a tiny mite, less than 0.4 mm long, that lives in your pores and hair follicles, usually on the nose, forehead, cheek, and chin, and often in the roots of your eyelashes.

(A follicle is the pore from which a hair grows). Demodicids have a wormlike appearance, with legs that are mere stumps. People with oily skin, or those who use cosmetics heavily and don't wash thoroughly, have the heaviest infestations ... but most adults carry a few demodicids. Inflammation and infection often result when large numbers of these mites congregate in a single follicle. The mites live head-down in a follicle, feeding on secretions and dead skin debris. At the left, you can see three demodicids buried in the follicle of a hair, and you can also see the hair's shaft. If too many mites have buried into the same follicle, it may cause the eyelash to fall out easily.

An individual female may lay up to 25 eggs in a single follicle, and as the mites grow, they become tightly packed. When mature, the mites leave the follicle, mate, and find a new follicle in which to lay their eggs. The whole cycle takes between 14 to 18 days.