Viruses are not just agents that cause disease. Throughout these pages, professor and science communicator José Antonio López Guerrero (JAL) offers a revolutionary and fascinating view of these nano-organisms that have shaped the evolution of life on our planet. Discover the viral side of evolution. Did you know that some viruses were crucial for the existence of placental mammals? Or that bacteriophages could be our last hope against antibiotic-resistant bacteria? Have you ever wondered what role viruses play in our planet's climate? Drawing on his experience as a researcher and science communicator, the author takes us on an exciting journey that transforms our perception of viruses: from their role in the development of the vertebrate brain to their use in cancer therapies, including the most innovative applications in biotechnology and medicine. This book reveals the most surprising and beneficial side of these tiny traveling companions, without whom life as we know it would not exist. An essential book for understanding that not all viruses are enemies and that many of them could be our best allies.