The causative agent of smallpox is variola virus and genus Orthopoxvirus.
There are Other members that can infect humans as vaccinia virus, monkeypox virus, and cowpox virus.
These viruses can be transmitted after the rash develops by face to face contact through the saliva by coughing, sneezing, or speaking of infected person or contact with his body fluids as fluids from blisters of skin.
It also can be transmitted by inhalation of large, virus-containing airborne droplets of saliva from an infected person that contain airborne virus.
Also, transmition can be occurred by touching contaminated items with the variola virus (via scabs or fluid like blankets, towels, or clothes as the virus can survive outside the body for up to 24 hours.