Viruses are amazing creatures. They are the most common, the most diverse, and the fastest-evolving biological entities on Earth. They infect every form of life known, “hijacking” the complex machinery of cells and forcing them into submission. Being much smaller and less complex than cells, they have a unique, tiny kit of “tools” able to regulate the essential elements of cells and to “fool” their defense mechanisms. It should be noted that viruses do not exhibit any of the life properties we usually attribute to cells (such as metabolism, development, or sensitivity) other than reproduction. What viruses practically “do” is to enter cells, their “hosts,” and use the cellular machinery to produce new virus particles. It is not surprising that many important discoveries in biology during the last 100 years have been made from, and through, viruses. Viruses have provided fundamental clues to the principles of molecular biology, such as how cells replicate and handle their information and the mechanisms that cause cancers, among many others.