This document provides methods and materials related to the use of nucleic acid coding for viruses to reduce the number of viable cancer cells within a mammal. For example, methods for using infectious nucleic acid to treat cancer, engineered viral nucleic acid, methods for making engineered viral nucleic acid, methods for identifying infectious nucleic acid for treating cancer, methods and materials for controlling virus-mediated cell lysis, and methods and materials for assessing the control of virus-mediated cell lysis are provided.