The Introduction outlines the structure of this VSI. The first two chapters introduce viruses, their structure and diversity, how they live, and their effects. Then the constant battle between viruses and the immune system of the infected individual is outlined, followed by chapters about infection by emerging viruses, epidemic viruses, pandemic viruses, and those that persist in the body for a lifetime. Later chapters look at how our knowledge of viruses has advanced through the ages and how the recent molecular revolution has enhanced our ability to isolate new viruses and to diagnose and treat virus infections. The final chapter speculates about how humans and viruses might interact in the future.