Poxviruses that cause disease in humans belong to four different genera of the vertebra poxvirus subfamily. The majority these viruses belong to the genus Orthopoxvirus.
Poxviruses were first visualized by electron microscopy in 1938. The genus Orthopoxviruses comprises many related viruses and includes, in addition to variola major, vaccinia, virus, cowpox, monkeypox, ectromelia, camelpox, taterapox, Uasin Gishu disease and racoonpox.
The orthopoxvirus Variola virus is the only member species of the Orthopoxvirus genus which is a solely human pathogen.
Variola virus, like all orthopoxviruses, replicates solely in the cytoplasm of infected cells. At least two types of Variola virus have been clearly described that have distinct biological and genetic properties and different human clinical and epidemiological manifestation.
These have been described as variola major and Variola minor. Another variant has been suggested, intermedius.
The virion of Variola virus has the same size and shape as that of all other orthopoxviruses.
The most important property of variola virus, in relations its effects on the human host, is its virulence. Some epidemics of smallpox were associated with high mortality and other with a low mortality.
The genetic difference between the variola strains most likely reflects differences in their replication and host-cell assembly leading to a greater or lesser virulence.
Variola virus of smallpox
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