Book excerpt: A fascinating and long overdue examination of viruses - from what they are and what they do, to the vital role they have played in human history.What are viruses? Do they rely on genes, like all other forms of life? Do they follow the same patterns of evolution as plants and animals?Dr. Frank Ryan answers these questions and many more in a sweeping tour of illnesses caused by viruses. He examines the common cold, measles, chicken pox, herpes, mumps, and rubella, as well as less familiar maladies, such as rabies, "breakbone fever," hemorrhagic fevers like Ebola, and virus-induced cancers. Along the way, readers will learn about the behaviors of viruses and what drives them to infect a human host. Ryan explains the role of viruses in the evolution of life, revealing how viruses have changed us at the most intimate level, helping to make us quintessentially human.