Viruses are the world's most abundant life form, and now, when humanity is in the midst of a close encounter with their immense power, perhaps the most feared. But do we understand viruses? Possibly the most enigmatic of living things, they are sometimes not considered a life form at all. Everything about them is extreme, including the reactions they evoke. However, for every truism about viruses, the opposite is also often true. So complex and diverse is the world of viruses that it merits being labelled an empire unto itself. And whether we see them as alive or dead, as life-threatening or life-affirming, there is an ineluctable beauty, even a certain elegance, in the way viruses go about their lives-or so Pranay Lal tells us in Invisible Empire: The Natural History of Viruses.