We are not alone. We are home to a trillion cells that are not our own, but are very much the making of us. Both on us and inside us live bacteria, viruses, protozoans, fungi, worms, lice and mites that we carry throughout our lives. To say that we are in a minority in our own body is an understatement. Our ‘private wildlife’ keeps us healthy, sometimes makes us ill and even changes our behaviour. Our journey of discovery through the natural history of bugs begins with the skin. We are all covered in resident bacteria providing our first line of defence against invading pathogens. Even before our immune system is activated, this protective layer of bacteria will fight off alien species. But if the skin is compromised, these invaders can threaten our lives.