A viral genome is the complete set of genetic material found in a virus. It can be made up of either DNA or RNA, depending on the type of virus. This genetic material contains the instructions necessary for the virus to replicate and produce new virus particles. The structure of a viral genome can vary widely, ranging from single-stranded to double-stranded forms. When a virus infects a host cell, it injects its genome into the cell, hijacking the cell's machinery to replicate itself. Understanding viral genomes is crucial for developing vaccines and treatments, as it helps scientists study how viruses like influenza or HIV evolve and spread.