Viruses pose a huge burden on public health systems and the economy worldwide. In the case of humans, viruses make up about two-thirds of all new human pathogens. Both DNA and RNA viruses can cause acute, persistent, or latent infections, as they can integrate their genome into the host DNA, such as the endogenous retroviruses, which can be associated with a wide variety of cancers in animals and humans. In recent years, viruses have emerged not only as a health threat but also as a potential alternative for producing new diagnostic tools as well as prophylactic/therapeutic approaches to defeat viral diseases. This book provides a broadly comprehensive series of reviews describing the replication strategies used for both DNA and RNA viruses, their interaction with the host immune system, and their potential role in tumorigenicity and cancer immunotherapy as applied to various viruses of critical relevance for human or animal diseases.