Smallpox was a contagious disease caused by the variola virus. Four orthopoxviruses cause infection in humans&colon; variola, vaccinia, cowpox, and monkeypox. Smallpox is a highly contagious disease caused by the Variola virus. Smallpox is caused by either of the two viruses, Variola major and Variola minor. The two classic varieties of smallpox are variola major and variola minor. Following the eradication of the human-specific variola virus (smallpox), all human "Orthopoxvirus" infections are zoonoses. The title refers to the "Variola vera" virus which causes smallpox. Cowpox is a poxvirus in the same family as variola. In 1721, Boston experienced its worst outbreak of smallpox (also known as variola). Moreover, shortly after turning to medicine, Hartley became an advocate of variola inoculation for smallpox. Half of the deaths were reportedly due to an outbreak of variola (smallpox) in 1863. House demands they administer the treatment for the smallpox variola. Variola virus and Camelpox virus form a subgroup. This form of variola major was more easily confused with chickenpox.