It is known that viruses contain DNA or RNA- either one and not both.
I came across a question: Which virus contains both DNA and RNA?
Here is the results summary of the study that describes the discovery of DNA:RNA hybrid virus:
Results Bioinformatic analysis of viral metagenomic sequences derived from a hot, acidic lake revealed a circular, putatively single-stranded DNA virus encoding a major capsid protein similar to those found only in single-stranded RNA viruses. The presence and circular configuration of the complete virus genome was confirmed by inverse PCR amplification from native DNA extracted from lake sediment. The virus genome appears to be the result of a RNA-DNA recombination event between two ostensibly unrelated virus groups. Environmental sequence databases were examined for homologous genes arranged in similar configurations and three similar putative virus genomes from marine environments were identified. This result indicates the existence of a widespread but previously undetected group of viruses.
And here is the link to the paper:
Hepatitis B virus (and other hepadnaviruses) have mainly double-stranded DNA genomes, but include a short piece of RNA
Genome: The HBV virion genome is circular and approximately 3.2 kb in size and consists of DNA that is mostly double stranded. ... The other strand, the plus strand, is variable in length, but has less than unit length, and has an RNA oligonucleotide at its 5' end.
Unlike others, HBV virions contain both DNA and RNA. Moreover, some regions of the packaged genome can be single stranded, double stranded, or even triple stranded. These peculiar features are a direct result of the HBV genome replication mechanism which will be covered later in this chapter.
However, this is just a short piece of RNA that's used to prime genomic replication.
Most virions have either RNA or DNA - never both. The exception to this rule is human cytomegalovirus (HCMV). In 2000, researchers reported that although HCMV contains mainly DNA, it also carries four pieces of RNA that are used to synthesize viral proteins.
Microbiology: An Introduction. Barry L. Batzing