Here's why:

* DNA viruses: Many viruses have their genetic material in the form of DNA. This DNA can be single-stranded or double-stranded and can be linear or circular.

* RNA viruses: Some viruses, like the influenza virus or HIV, use RNA as their genetic material. This RNA can also be single-stranded or double-stranded.

So, while some viruses use RNA as their genetic material, not all viruses do.