It's enough to make your skin crawl: the thought of living one's life completely surrounded by bugs and bacteria. Some of these organisms live off of the structure of your house, gnawing, digesting, and converting to sawdust what was once a solid structure. Other organisms live on your pets, especially in their furry coats. And, believe it or not, countless organisms -- in fact, billions of creatures -- live on the surface and inside of your body, most of the time without your knowing.

How do all these uninvited guests go unnoticed? Although many of these organisms live out in the open -- on the back of your hand, for instance -- most are simply too small to be seen. No matter how carefully you look, it is impossible to see the dust mites and bacteria that inhabit your skin without the aid of a microscope. As for the larger organisms, including carpenter ants, termites, silverfish, and a host of others, these creatures are notoriously good hiders, staying concealed for most of their lives or coming out only under cover of darkness.

It may be uncomfortable to think of your home and especially your body as habitats for other animals, but that's exactly what they are. These organisms share our space for the same reasons other animals live in a forest ecosystem or a river ecosystem: because they find food and shelter there. Wooden window and door frames provide protection and food for carpenter ants and house borer beetles, just as our skin and digestive system provide the same for mites and bacteria.

Thankfully, most of the creatures that use our homes and bodies for habitats are relatively harmless. In fact, some of these organisms provide a service to us and our environment, much like the role decomposers play in a forest ecosystem. Dust mites, for example, consume the dead skin cells that our bodies slough off at a rate of about 1.5 million per hour. In turn, our discarded cells provide the mites with an energy-rich food, one that might otherwise go unused. Bacteria in our stomachs and intestines also help us to break down food that is difficult to digest and also fight off harmful bacteria that invade our bodies.

Of course, knowing the good these organisms do doesn't make them any more welcome. But the next time you feel creeped out by what's crawling on your dog, just remember he could say the same about you.