杏吧原创

Last night, while you were sleeping, legions of eight-legged creatures had an orgy between your eyebrows. No, you haven鈥檛 suddenly been invaded by sex tourists. Demodex mites, close relatives of ticks and spiders, are permanent and mostly harmless residents of the human face.

鈥淓very person we鈥檝e looked at, we鈥檝e found evidence of face mites,鈥� says at North Carolina State University in Raleigh. 鈥淵ou can have thousands living on you and never even know they鈥檙e there.鈥�

Growing up to 0.4 millimetres long, these beasts spend their days buried head-down in hair follicles gorging on who-knows-what and crawling out under cover of darkness to copulate. They have no anus, so on death disgorge a lifetime of faeces into your pores.

Before you lunge for the exfoliating brush: Demodex mites are far from your only microscopic residents. You host astonishing biodiversity, from anus-less arthropods to pubic lice to all manner of bacteria and fungi, and without it you wouldn鈥檛 be who you are. 鈥淓ach of us is really a complex consortium of different organisms, one of which is human,鈥� says at Stanford University in California.

Our resident aliens aren鈥檛 all benign. There are big beasts like parasitic worms: roundworm, hookworm and whipworm are prevalent in the developing world, and pinworm still infects kids in the West. Then there are hidden viruses such as Herpes simplex, which lies dormant inside the nerve cells of two-thirds of people until it mistakes your sniffles for a deadly fever and attempts to save itself by rushing outwards, causing cold sores.

鈥淟ast night, legions of eight-legged creatures had an orgy on your face鈥�

By far the dominant group, however, are bacteria. You have , perhaps 10 times more. Only recently have we begun to grasp the extent of their diversity, and there鈥檚 plenty left to discover. This year we found the first bacteria that survive by parasitising other bacteria. They live in your spit.

Similar battles play out across your many habitats, from the caves of your nostrils and your anal-genital badlands to the crevices between your toes where the fungus Trichophyton rubrum can flare up as athlete鈥檚 foot. All of these critters are constantly shedding from your skin and lungs, forming your own unique cloud of airborne bacteria that follows you everywhere.

But the densest microbial gathering is in our gut, a community that affects aspects of health from digestion and immune defences to possibly even mood and behaviour. In mice, seeding the gut with Lactobacillus rhamnosus bacteria has been shown to , perhaps by producing molecules that alter brain chemistry.

The balance of gut microbiota can shift rapidly in response to diet and lifestyle. To tend it you need to feed it right. Your best bet isn鈥檛 much-hyped probiotics or live bacteria, but simply to eat more fibre, the preferred meal for a group of bacteria with potent anti-inflammatory powers. 鈥淚t has been known for a long time that plant-based fibre is associated with good health,鈥� says Sonnenburg. 鈥淣ow we know why.鈥�

This article appeared in print under the headline 鈥淵ou are鈥� a menagerie鈥�