The smallpox virus
- also called poxvirus variolae - is the cause of smallpox.
In the case of naturally occuring smallpox (ie. Not including bioterrorism) the virus can only be spread from another person with smallpox. No cases of smallpox have existed anywhere in the world since the late 1970's. In general, it is believed that the only possible cause of a new smallpox outbreak would be bioterrorism.