This introductory chapter provides an overview of how viruses have caused geographic, economic, and religious changes. Smallpox alone, in the twentieth century, killed an estimated 300 million individuals, about threefold as many persons as all the wars of that century. In the sixteenth and seventeenth centuries, smallpox killed emperors of Japan and Burma as well as kings and queens of Europe, thereby unseating dynasties, altering control of countries, and disrupting alliances. In addition to propelling the establishment of Christianity in Mexico and Latin America, viruses played a role in enlarging the African slave trade throughout the Americas. In contrast to viruses such as smallpox and measles which are now harnessed by the innovations of healthcare, new viral plagues of fearful proportions have appeared. These include HIV/AIDS, sudden acute respiratory syndrome, Ebola, Zika, and bird flu. This book looks at the history of viruses and virology, which is also the history of the men and women who have worked to combat these diseases.