Why Viral Nucleic acid (Genome) is unique?

During high school you might have learned two very basic rules about nucleic acid: DNA and RNA. Deoxyribonucleic acid (DNA) is a double stranded molecule, i.e. it consists of two strand of DNA bound together and RNA is a single stranded molecule.

But this rule may not hold true in case of viruses. Viruses are only creatures that have genetic material composed of single stranded DNA / double stranded RNA.

1. Viruses having double stranded RNA as their genome: virus of family Reoviridae (Rotavirus, Colorado tick fever virus) and Birnaviridae have double stranded RNA as their genome.
2. Virus having single stranded DNA as their genome: virus of family Parvoviridae (Parvovirus B-19) possesses single stranded DNA.

Other unique properties of viral genomes

1. Viral genomes are either DNA or RNA but not both
2. Nucleic acid can be either linear or circular
3. RNA can exist in several pieces. Influenza virus and rotavirus have segmented RNA genome
4. Almost all viruses are haploid, i.e. they contains single copy of genome with a major exception of retrovirus family which is diploid (have two copies of their RNA genome).