We live in a viral world. We are continually exposed to viruses in our environment, and some viruses are even part of our own genomes. Fortunately, most viruses are innocuous although some can sicken us, some can cripple or scar us, and some can kill us with frightening destruction of our bodies. Among the many types of viruses that infect humans, their physical and biological properties vary greatly. Yet they all have one thing in common—once they infect us they are difficult to treat. In many cases, the only option is to provide supportive or palliative care and hope that our immune system defeats the viral invader before we are ravaged or killed by the infection. Unlike the widely available antibiotics for bacterial infections, until recent decades there were no effective drugs to treat any viral infections. Much progress has been made in antiviral drug development in recent years, yet we still lack antivirals against many of our most important viral pathogens. Our only defense against many viruses is to prevent infections from happening through prophylactic vaccination. Consequently, much of twentieth-century virological research was devoted to developing vaccines against dangerous endemic viruses that routinely swept through the global population each year. These efforts were an unqualified success and many once common viral diseases virtually disappeared from any nation capable of widely vaccinating its populace. Vaccines are widely acknowledged as one of the most successful medical advances in the twentieth century, preventing hundreds of millions of deaths worldwide.