Contagion with variola minor confers immunity against its more dangerous form, variola major.
Like all forms of smallpox it is caused by the variola virus.
There is no evidence of chronic or recurrent infection with variola virus.
Camelpox, taterapox and variola viruses arose 3,500 years ago and horsepox virus 3,000.
Variola minor is a less common form of the virus, and much less deadly.
Variola major was the severe and most common form, with a more extensive rash and higher fever.
Variola virus infects only humans in nature, although primates and other animals have been infected in a laboratory setting.
Negri went on, however, to demonstrate in 1906 that the smallpox vaccine, then known as vaccine virus, or variola vaccinae, was also a filterable virus.
