Viruses are probably the most common infectious agents, though no exact figures exist to support this contention. As a group, they are all obligate parasites and infect not only animals and plants but also other microorganisms. Every time a new virus or viral disease is discovered or a connection is made between a virus and a once-unexplained disease, our understanding of these remarkable infectious particles is increased, and we are reminded of their power. This chapter covers viral diseases of the viral groups most significant to humans and their epidemiology, pathology, and methods of control, with special emphasis on DNA viruses. Chapter 25 covers RNA viruses.