The smallpox disease is cause by the Variola Major and Variola Minor (Alastrim) viruses.  Smallpox belongs to the Orthopox family, along with monkeypox, cowpox, camelpox, vaccinia and others.  Infection with any one of this family of viruses is considered to provide immune protection to all of them.  This is due to the conserved nature of immunogenic proteins on the surface of these viruses..

Dangerous

Live variola virus (smallpox) can only be stored and studies in either one of two locations in the world - the CDC in Atlanta and the Vector lab in Russia.  These labs handle variola under BSL-4 containment and only by vaccinated personnel.

Ancient

There is evidence of smallpox mortality in ancient Egypt.  The first examples of artificially-induced immunity is variolation in China.  Later, Edward Jenner would use "cowpox" as the first ever vaccine to create immunity in humans.

Eradicated

Variola was declared eradicated in the world by the World Health Organization in 1980.  This was the first virus ever deliberately eradicated from the human population.  Samples of the virus were kept for study in maximum containment labs.