Apparently they are completely harmless and we've even known about these critters - known as Demodex folliculorum and Demodex brevis - for over a hundred years, but we are only just starting to understand them now, according to Wired.
The Dunn lab at North Carolina State University has been studying the mites for years and has taken hundreds of samples from all different types of people - you can see a handy/gross gallery of their findings here.
Although the fact that neither species has an anus, and therefore cannot defecate, may seem like a good thing, it actually makes everything worse.
As Wired explains, instead of defecating as they go, the little mites just store up all their waste until they die. After which their grip on your pores is loosened, they are released onto the surface and their waste becomes a part of the oily layer keeping your skin moist.