Читать книгу Virusphere - Frank Ryan - Страница 8

Only in the last decade have we come to realise that, from its very beginnings, all of cellular life has inhabited not only the visible biosphere – of solid earth, air and oceans – but also a less familiar and invisible virosphere. The viruses that constitute this virosphere are not merely surrounding us, they are within us, both as evolving extrinsic organisms in themselves and as interactive symbiotic entities that are an intrinsic part of our being. We might not be aware of the presence of these minuscule passengers within us from moment to moment, but the passengers are, in their quintessentially viral way, aware of us.

This might seem somewhat daunting, even frightening, to some of us, but there is no need for alarm. They have always been there. It is likely that they preceded any origins of human life on planet Earth, or indeed, going further back, the origins of the mammals, or any animals or plants, or fungi, or, if I am right, even the single-celled amoebae. All that has changed is that the world of virology is coming to understand the role of viruses in the origins and diversity of life, and what might appear incongruous to any notion of viruses being exclusively agents of disease, the health of the biosphere.

For viruses to achieve all this they must surely possess some remarkable properties. For example, they have no means of locomotion yet they move among us: in pandemic forms they effortlessly circulate around the globe. Although they have no sense of vision, hearing, touch, smell or taste, they detect with uncanny precision the cell, or organ or tissue that is their target destination. This they achieve despite the relentless opposition of powerful immune defences designed to prevent this happening; and once arrived, they penetrate the defences of the target cell, break entry through its protective surface membrane, and once inside take over its physiological, biochemical and genetic programming to compel the cell to become a factory for the production of a new generation of themselves.

Welcome to the world of viruses!

It is, admittedly, a very strange world replete with mysteries. It becomes all the more quixotic when we attempt to examine it at the most basic level.

What then are viruses? How do we even begin to define them? What, for instance, is the difference between, say, a bacterium and a virus? While viruses and bacteria are often confused in the minds of ordinary folk, because they cause many of the common infectious diseases, bacteria and viruses are very different entities. Viruses are more difficult to define than bacteria because they are said to occupy a position somewhere between the biological notions of life and non-living chemicals. This has tempted a distinguished colleague to dismiss them as ‘a piece of mischief wrapped up in a protein’. While the hubris contains a grain of truth, there is rather more to viruses than being merely a source of mischief. So let us delve a little deeper! Do viruses rely on genes, and genomes, like all of the more familiar forms of life, from whales to humans and buttercups to the so-called ‘humble’ bacterium? The answer to that question is, ‘Yes!’ Viruses do indeed have genomes, which contain protein-coding genes. We shall discover more about those viral genomes in subsequent chapters when we shall also observe some important differences between the genomes of viruses and all other organisms.

Do viruses follow the same patterns of evolution as, say, plants and animals? The answer again is, ‘Yes!’ But the patterns of evolution – the specific mechanisms involved – are heavily influenced by a facet of their organismal existence that is confined to viruses. Viruses can only replicate by making use of the host cell’s genetic apparatus, and because of this, viruses were formerly defined as ‘obligate genetic parasites’. But with our increasing understanding of viruses, and of their complex roles in relation to the evolution of their hosts, this definition is no longer sufficient to characterise them. A more adequate definition must take on board the fact that viruses are symbionts. Indeed, we now know that viruses are the ultimate symbionts, exhibiting many examples of all three patterns of symbiotic behaviour, namely parasitism, commensalism and mutualism. Moreover, since viruses will sometimes employ aggression as an evolutionary pattern of behaviour in relation to their hosts, they are also potentially ‘aggressive symbionts’.

The more we examine viruses, in their evolutionary trajectories and in the influence of that trajectory on the evolution of their hosts, the stranger and more fascinating their story becomes. Is it reasonable to propose that viruses were born at the stage of chemical self-replicators before the actual advent of cellular life on Earth? If so, how then, from those primal beginnings, did viruses evolve, to interact with, and thus contribute to, the evolution of all of life on this planet?

The aim of this book will be to enlighten readers through a step-wise progression, starting with a familiar territory: we shall confront the wide range of illnesses that are caused by viruses. For example, we shall examine what is really going on in the common cold, the childhood illnesses such as measles, chickenpox, herpes and mumps, rubella, as well as less familiar examples such as rabies, ‘breakbone fever’, haemorrhagic fevers such as Ebola, and virus-induced cancers such as Burkitt’s lymphoma. In such an examination we shall discover what makes viruses tick, exploring what’s actually going on inside us when we encounter the virus, how this gives rise to the symptoms we get from the infection, and, key to deeper understanding, probing what the virus itself gets from the ‘interaction’ with its human host. We shall employ the same virus-orientated perspective to explore important epidemic forms such as influenza, smallpox, AIDS and polio, which will illustrate how viral infections have impacted on human social history, from the wall paintings of the Ancient Egyptians to the colonisations of the Americas, Australasia and Africa. We shall also take a close look at vaccines as a measure to prevent epidemic infections, from the first introduction of vaccination against smallpox centuries ago to the recent controversy concerning the triple vaccine and the papilloma virus vaccine.

The science of virology grew out of the study of viruses in the causation of disease. Through understanding the viruses already familiar to us, we shall widen our enlightenment by examining the role of viruses in the evolution of life, and in particular we shall explore the role of viruses in our human evolutionary history. We shall see how, throughout our prior evolution, we have shared our existence with these powerful invisible entities, and how they really have changed us at the most intimate level, to help make us human.

I hope that, like me, you will come to appreciate the enormous importance of viruses to life, in its origins and complexity, while also marvelling at the existential nature of what is one of the great wonders of life on our beautiful blue-oceanic planet. Viruses, by and large, have had a bad press. This is understandable, given the experiences of earlier generations of virologists, whose only contact with viruses was in dealing with the infections caused by them. But today a major wind of change is blowing through the world of virology – so much so that recently a distinguished evolutionary virologist declared that we were witnessing what he called ‘The Great Virus Comeback’. What does he mean by this? Why have some of the modern pioneers of virology introduced the term ‘virosphere’ as the key to a new exploration of the importance of viruses to the entire biosphere? Could it be true that, as some would have us believe, viruses should now be seen as the ‘Fourth Domain of Life’?