Here's why:

* Definition of a virus: Viruses are obligate intracellular parasites, meaning they need a host cell to reproduce. They are essentially genetic material (DNA or RNA) enclosed in a protein coat (capsid).

* The genome's role: The genome carries the genetic information that instructs the virus to:

* Replicate itself within the host cell.

* Produce proteins necessary for its survival and propagation.

* Diversity in genomes: While all viruses have genomes, they can differ significantly in:

* Type of genetic material: DNA or RNA

* Structure: Single-stranded or double-stranded

* Size and complexity: From small RNA viruses to large DNA viruses

Therefore, the presence of a genome is a defining characteristic of all viruses.