You probably have a routine you go through when you go to bed at the end of the evening. Maybe you have a milky drink or a bath. You probably clean your teeth, put the cat out, or set the alarm clock. Perhaps a little light reading before you switch off the bedside lamp, yawn, say goodnight to anyone sharing your room, then close your eyes and snuggle down to drift into a pleasant unconsciousness for maybe seven or eight hours. But tonight, while you dream, eight-legged creatures you may not have known even existed will emerge from their lairs and crawl around - all over your face.

There, that's one way to ensure you lie awake tonight, but in fact, although undoubtedly true, it's not quite as disturbing as it sounds. You'll eventually get used to it. You see, they're very small - maybe a few tenths of a millimetre in length. They're parasites, and you could go through your whole life without being noticeably disturbed by them in any way - well, if we hadn't told you about them, that is.

There are two species of face mite1: Demodex Folliculorum lives in your hair follicles - in fact, each follicle can support a small colony of around ten mites. The shorter Demodex Brevis prefers to live on its own in one of your sebaceous glands, which lubricate your hair and skin with an oily secretion. They're arachnids, but they don't look a lot like spiders, ticks or indeed other mites. Both species have scaly, elongated bodies, and eight short, stumpy legs with tiny claws; they move pretty slowly - around one centimetre per hour.

But don't worry - they don't bite! Well, not bite exactly, you see they have these needle-sharp mouthparts which they inject into your dead skin cells, before sucking up the cytoplasmic juice. More of a sting, really, but again, you shouldn't feel a thing, and here's the clever thing - they don't poo on your face. Clearing away dead skin cells without leaving unwanted by-products means they can do you more good than harm in this respect.

They do have sex on your face, though, and the female will lay up to 25 eggs in a follicle, which hatch into larvae, as eggs do. The life-cycle is between 14 and 18 days, in case you were taking notes.

We aren't born with face mites, but acquire them over the course of our lives. At the age of 20, it has been estimated that 25% of us have them, with this figure increasing to 100% if we live to 90. We pick them up from using other people's combs and hats, from things like headrests, and of course from close contact with people. Armed with this knowledge, the more squeamish of you might wish to stick to air-kisses with Granny from now on.

Like humans, they are mostly harmless, but it's only fair to tell you that they have been implicated in a number of minor skin diseases. A large infestation of them can result in an itchy condition known as demodicosis - if you were a dog, the vet would call it 'red mange'. They can also contribute to hair loss: they secrete the enzyme lipase to help digest sebum - the oil secreted by the sebaceous glands - and this can clog up the hair follicles. Of course, if you suspect you are suffering from any condition, then contact a medical professional for advice.

If you weren't previously aware of hosting face mites, then we apologise for letting this particular genie out of the bottle. Remember, however, that even an infestation of these is nothing compared to some of the parasites you could pick up: bott fly maggots, tapeworms, even the Amazonian candiru fish which swims up... no, let's not go there.