Are you ever really alone? Even when you think you are sitting at home alone, you are still in the company of a great number of living things. Bacteria live on you and in your body, and constantly protect you and also, occasionally, cause you to become ill. Mites that are spread everywhere, from the chair you sit in to your carpet, to the air you breathe. Moulds and fungi begin reproducing on foods left out in the open in your kitchen for even a few hours—all of these constitute a different world with its own unique lifestyles, nutritional systems and structural features. Maybe you have always thought that humans, animals and plants you see around you represent the sole community of living things. Yet the microorganisms, members of a secret world that reach to every corner of the Earth, are far more numerous that those other, more familiar living things. These minute creatures outnumber the animals in the world by twenty to one. 1 In the same way that they are present all over the world, they are also essential to human life.