https://en.m.wikipedia.org/wiki/Smallpox

https://en.m.wikipedia.org/wiki/Orthopoxvirus

https://en.m.wikipedia.org/wiki/Poxviridae

https://en.m.wikipedia.org/wiki/Giant_virus

https://en.m.wikipedia.org/wiki/Nucleocytoviricota

https://en.m.wikipedia.org/wiki/Genome_size#Genome_reduction

https://en.m.wikipedia.org/wiki/Vertical_transmission

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Poxviridae
Orthopoxvirus

Phylum: Nucleocytoviricota
Orthopox: Smallpox

Nucleocytoviricota is a phylum of viruses. Members of the phylum are also known as the nucleocytoplasmic large DNA viruses (NCLDV).

The phylum is notable for containing the giant viruses.

Host organisms typically include protozoa, invertebrates and eukaryotic algae, via vertical transmission.

large DNA viruses may have evolved from a shared cellular genome ancestor by means of genome reduction.

The two main hypotheses are that they evolved from small viruses by picking up DNA from host organisms; or that they evolved from very complicated organisms via genome reduction, losing various functions including self-reproduction.