When I reviewed Planet of Microbes: The Perils and Potential of Earth’s Essential Life Forms, I remarked that microbes are everywhere. If you are willing to stretch the definition of life a bit further still, there is one entity that is even more numerous and omnipresent: the humble virus. We tend to think of viruses almost exclusively in the context of disease (see for example The Invisible Enemy: A Natural History of Viruses). But, as virologist and pharmaceutical researcher Michael Cordingley shows here, they are so much more than mere pathogens and have a huge influence on evolutionary processes in all organisms. This book paints a remarkable portrait of these unusual life forms.