There were two strains of smallpox, variola major and variola minor.

Contagion with variola minor confers immunity against its more dangerous form, variola major.

There are two stages of the smallpox virus, the Variola Major and the Variola Minor.

Those contracting Variola Minor had a greatly reduced risk of death - 1-2% - compared to those contracting Variola Major with 30% mortality.

Variola Major (Smallpox) Variola Minor (Alastrim) Smallpox is an infectious disease caused by the variola virus.

The earliest recorded artificial induction of immunity in humans was by variolation or inoculation, which is the controlled infection of a subject with a less lethal natural form of smallpox (known as Variola Minor) to make him or her immune to re-infection with the more lethal natural form, Variola Major.