This chapter provides an overview of the numerous patterns of different virus infections from the perspective of host organisms, and describes the major responses of plants and animals to virus infections. The scientific basis for prevention and treatment of virus diseases are also explained. Virus infection is a complex, multistage interaction between the virus and the host organism. The patterns of virus infection can be divided into these types: abortive, acute, chronic, pertinent, and latent. The course and eventual outcome of any infection are the results of a balance between host and virus processes. The host factors involved include exposure to different routes of virus transmission and the control of virus replication by the immune response. Virus processes include the initial infection of the host, its spread throughout the host, and the regulation of gene expression to evade the immune response. The chapter also discusses the virus infections of plants, immune responses to virus infections in animals, apoptosis (programmed cell death), interferons (the blocking of virus infection), and evasion of immune responses. Medical intervention against virus infections includes the use of vaccines to stimulate the immune response and drugs to inhibit virus replication. Molecular biology is stimulating the production of a new generation of antiviral drugs and vaccines.