Smallpox is caused by a virus called variola, which is a member of the poxvirus family. Poxviruses are large, complex viruses that can cause a range of diseases in animals and humans. Smallpox is a highly contagious disease that was once widespread throughout the world, but has now been eradicated through a global vaccination campaign. The virus spreads from person to person through the air, and can cause a rash, high fever, and other symptoms.