Two Demodex species live on humans: folliculorum in hair follicles and brevis in sebaceous glands. Detection rates rise steeply with age, and using sensitive molecular methods researchers have found the mites on essentially all adults sampled. Transmission is by direct contact, most likely from parent to infant.
Genomic work published in 2022 found D. folliculorum has an unusually reduced genome, having shed genes for ultraviolet protection and for making certain compounds it can obtain from the host. It has few cells, a simplified body, and lives in near-total isolation within a follicle, which limits opportunities for gene exchange. The authors described it as being on a trajectory from parasite toward internal symbiont.
The mites are a normal part of the human skin fauna and are not, in themselves, a disease. Elevated densities are associated with conditions such as rosacea and blepharitis, but whether the mites drive those conditions or simply proliferate in skin already altered by them is not settled.