Vaccinia is a virus closely related to cowpox and to the variola viruses that cause smallpox in humans. It is used to vaccinate people against smallpox. Today only laboratory workers and military personnel are vaccinated, now that smallpox has been eliminated in the wild. Vaccinia, variola, and cowpox belong to the orthopox genus Orthopoxvirus in the poxvirus family Poxviridae. Vaccinia is the type species of the genus.
The word vaccinia originally meant the cowpox virus itself, from the Latin vacca, a cow. The physician Edward Jenner learnt the folk tradition that anyone who had suffered from the mild cowpox disease was immune to smallpox.
The idea of inoculation had already been introduced into England from Turkey in 1721 by Lady Mary Wortley Montagu, but this involved the risky practice of using a mild form of smallpox as the immunizing agent, a practice now known as variolation. Of the two variola viruses, variola minor causes less than 1% mortality, compared to 30% for variola major, so it was important to get the right strain. Of course the idea of strains and viruses was unknown in those days. It was Jenner who later coined the word virus. On 14 May 1796 he took a cowpox pustule from a milkmaid named Sarah Nelmes and inoculated eight-year-old James Phipps with it. A few weeks later he exposed Phipps to smallpox, and the boy was unaffected, and thus vaccination was proved effective.
So vaccinia was originally thought to be cowpox. The two viruses are now clearly distinct, and it is not known how this has come about. It could be a mutated form of smallpox (variola) that lived on cows, it could be a mutated cowpox, or it could be a hybrid. It infects a wide range of host species, not just humans and cows, but has no known natural reservoir.
It is not harmless. It is itself an infectious virus, and can cause complications after immunization, and can be passed from immunized to non-immunized people. This can be very serious for people with eczema or who have a compromised immune system.
Its genome has been sequenced, and it has 191636 nucleotides. The virion (virus particle) is somewhat brick-shaped, with dimensions from 200 nm to 300 nm.
http://life.bio2.columbia.edu/ICTVdB/58110001.htm for description
http://www.bats.ch/abstr/197k2a13.htm for theories of origin, and for transmission
http://www.cdc.gov/mmwr/preview/mmwrhtml/00000556.htm for precautions