Book review / A plague on all our houses?

Plagues: Their Origin, History and Future by Christopher Wills HarperCollins, pounds 20; A new history of disease suggests that, paradoxically, viruses are essential for human survival.

Disease attracts metaphors like the plague. When Christopher Columbus first encountered the massive outflowing waters of the Orinoco, he was convinced it was one of the four rivers of Eden itself. When he set foot on land he was to set in motion a catastrophic chain of events: the fiery angel guarding the doors of paradise was disease. Columbus's men brought syphilis back to Europe and left fatal sicknesses like smallpox to wipe out the natives.

Metaphors are right up there with transposons and invasins as integral to the action of infectious disease, and perhaps we should forgive Christopher Wills for hauling out the trusty Rain Forest trope when trying to explain, towards the end of this interesting book, why viruses are essential to life's evolution and diversification.

Viruses are the joker in the DNA pack. Gardeners know that viruses not only kill their flowers and fruit trees, they can also remain more-or- less inert and inflict mere beautification - variegated leaves on holly bushes usually get their picturesque markings from the workings of a virus following some unfathomable machination of its own design. Plague pathogens, argues Wills, inhabit a beautiful garden of their own which only very rarely intersects with our existence.

It seems we have now accumulated so many pieces of virus-sourced DNA in our genes over millions of years that we are almost indistinguishable from disease itself. There are more than 10,000 retroviral fragments in human chromosomes, relics of long-forgotten viral battles of the past. Why it should be in our evolutionary interests to carry around this debris in the most intimate part of our humanity is unclear.

Plagues have always had weighty Biblical connotations. The virus is a paradox. It's at once very modern and very ancient, religious and scientific, organic and inorganic, metaphysical and bodily, and, while cultural commentators strive to find increasing links between this age and the medieval one, a very fashionable millennial fear. Something about plagues reminds us of our vulnerable animal selves, our unconsciousness; it's no wonder that AIDS, typhus, syphilis, malaria and even the Bubonic Plague have had animal vectors involved or suspected.

It seems we haven't had a true plague since the Black Death - only epidemics and pandemics. Even the plague in the London of Pepys and Defoe was far too irregular in intensity to be classified as a plague. Wills thinks it's unlikely we'll have another Black Death, despite the pressures of disease originating from overpopulation and access to pockets of viruses which have been breeding undisturbed for hundreds of thousands of years in crannies of the tropics. Why? With old-fashioned 18th-century positivism, Wills is still convinced that we can out-think the spread of a virus and that increasing population, far from increasing a tendency for wildfire diseases, cranks up levels of "herd immunity".

Evolution is really what this book is about. Theorists argue convincingly that most of our better adaptations have come about through stresses in an environment; Wills extrapolates into saying that disease is possibly the biggest source of stress in any species. But he doesn't really explain how the domino effect of evolution came about, and why it's necessarily a good thing.

Disease is an illuminating mirror to hold up against mankind, and Wills's discovery of a variegated genetic beauty underlying it all is a great feat of empathy. I find it an agreeable piece of syncretism that these feckless agents of death may have been powering the human drive into consciousness all along.