Recently I came to the conclusion that mites are one of the most under-appreciated groups. Still, I was shocked to find out about Demodex folliculorum, a weirdly elongate mite that lives only in the hair follicles of humans. Demodex brevis, possibly a sister species, lives only in our sebaceous glands. Adults max out at a whopping .4mm, small enough to allow many to lay eggs on your eyelids (apparently a preferred location) without you even knowing they existed. What are the chances you are infected by Demodex? It depends on your age. Young children are estimated to be infected 1/3 of the time while approximately half of adults are infected. Other than some minor skin irritation the mites cause almost no harm to their host.
First image: a colored SEM of Demodex.
Second image: Demodex life cycle described from human eyelid.
Third image: Demodex elongated abdomen visible at the base of a hair.
Second and third images from:
English, F. et al. (2006) The natural history of demodectic mites on the skin of the eyelid margin. Journal of the European Academy of Dermatology and Venereology. 2(2) 132-136