Viruses, for all intents and purposes, are invisible – we cannot see them, smell them or feel them. But viruses are everywhere; in the oceans, on the land, in the air, in plants and in our bodies, and those of all other animals. There are more viruses on Earth than all forms of life put together. We are only aware of their presence when they cause disease, but very few of the, probably, millions of different types of viruses actually do this. They have been around longer than humans – and pretty much everything else – and infected the first cells to have evolved on Earth. So bad is their reputation that it often comes as a surprise to discover that they are essential to life and have been a powerful force in evolution.
They were not discovered until relatively recently, and they have only been studied directly for the past 100 years. Before then, the diseases they caused were often thought to be caused by supernatural activity including the influence of the stars, and this is where the word "influenza" is derived from. Similar superstitions are still believed in some communities, which often make the control of the spread of viral infections difficult.
The 20th century was a golden age, and now much is known about viruses – what they are, what they do and where they came from. Despite their smallness, viruses are complex, and the way they reproduce in their host cells is even more so. This leads to difficulties in description because technical terms are often unavoidable. We have only one word for "cell" and, even more unfortunately, "messenger RNA". To have used definitions rather than such words, would have made the prose cumbersome. In the Wikipedia versions of the book's chapters, these terms are linked to other pages that explain them. This is difficult to achieve in print but will hopefully be addressed in later editions in the form of a glossary, when the corresponding Wikipedia article has been created. In the meantime, the reader might find my Wikipedia article Introduction to viruses useful.
The social history of viruses describes the influence of viruses and viral infections on human history. Epidemics caused by viruses began when human behaviour changed during the Neolithic period, around 12,000 years ago, when humans developed more densely populated agricultural communities. This allowed viruses to spread rapidly and subsequently to become endemic. Viruses of plants and livestock also increased, and as humans became dependent on agriculture and farming, diseases such as potyviruses of potatoes and rinderpest of cattle had devastating consequences.
Smallpox and measles viruses are among the oldest that infect humans. Having evolved from viruses that infected other animals, they first appeared in humans in Europe and North Africa thousands of years ago. The viruses were later carried to the New World by Europeans during the time of the Spanish Conquests, but the indigenous people had no natural resistance to the viruses and millions of them died during epidemics. Influenza pandemics have been recorded since 1580, and they have occurred with increasing frequency in subsequent centuries. The pandemic of 1918–19, in which 40–50 million died in less than a year, was one of the most devastating in history.
Louis Pasteur and Edward Jenner were the first to develop vaccines to protect against viral infections. The nature of viruses remained unknown until the invention of the electron microscope in the 1930s, when the science of virology gained momentum. In the 20th century many diseases both old and new were found to be caused by viruses. There were epidemics of poliomyelitis that were only controlled following the development of a vaccine in the 1950s. HIV is one of the most pathogenic new viruses to have emerged in centuries. Although scientific interest in them arose because of the diseases they cause, most viruses are beneficial. Retroviruses drive evolution by transferring genes across species and bacteriophages play important roles in ecosystems and are essential to life.