We’re used to thinking of bacteria as unwelcome hitchhikers. But in many cases, the boundary between the human body and bacteria is surprisingly blurred. First, your microbial partners have a hand in many of the functions that we consider a normal part of human life. Without beneficial bacteria, you’d lose valuable vitamins and give up your unique, lifelong body odor. You’d also have no defense against the attack of far more dangerous strains of bacteria and, as a result, you’d probably suffer from nearly constant diarrhea, skin rashes, and bladder infections. It’s even possible that without friendly bacteria to continuously prod your immune system, its careful calibration would slip, leaving you at a greater risk for allergies and asthma.
But bacteria are more than the permanent residents of your body. There’s solid scientific evidence that the relationship between humankind and bacteria is older and far more complex than most people realize. Many scientists now believe that mitochondria, the biological power plants that fuel the work of every human cell, were once free-floating bacteria that somehow became incorporated into the bodies of our most ancient, primitive ancestors. And almost all scientists agree that the tree of life leads back through time to a simple, single-celled creature that was at least a bit like a modern-day bacteria, which means that the stomach bug that makes you ill today might bear more than a passing resemblance to your great-great-great-great- (and so on) grandfather. Thank goodness no one had invented antibacterial soap back then.
Source of Information : Oreilly - Your Body Missing Manual