Today I learned that vaccinia is a thing. Would you like to entertainingly describe its attributes?
Vaccinia is a less harmful relative of the Variola virus (commonly known as SMALLPOX). Not only was it important in the eradication of smallpox, but it inspired vaccines against other viruses (How Vaccination Got Its Name) so you could call it the Traitor of Viruses Everywhere.
It is a similar enough to smallpox to generate lasting immunity, which was observed when people who had been exposed to cowpox did not seem to catch smallpox. Early smallpox immunizations were given by contact with material thought to contain Vaccinia (often pus from infected cows or from the scab that formed after another person was vaccinated) but in the 1900s it was grown and kept in labs.
Although vaccinia came from cowpox, now “cowpox” generally refers to the wild virus and “vaccinia” refers to lab-grown lines. They have diverged somewhat because controlled lab conditions are very different from the wild and it has likely affected the evolution of vaccinia. Cowpox is like a feral cat and vaccinia is a pet. (Smallpox is a tiger.)
Vaccinia is not harmless! It is able to reproduce within cells, and mild illness after smallpox vaccination is common. People with eczema or immune deficiencies should not receive it. There have been attempts to make smallpox vaccines with fewer side effects by modifying vaccinia to be incapable of reproduction. (They will probably not be needed, but useful in the event of not-so-soft bioterrorism.)
Cool fact about poxviruses in general: They are DNA viruses. Most DNA viruses replicate in the nucleus and use the host machinery to transcribe their DNA to RNA. Poxviruses replicate in the cytoplasm, so they carry their own transcription machinery.