Smallpox results from infection by variola virus (genus Orthopoxvirus, family Poxviridae). At least two strains of virus exist: the more virulent strain causes variola major and the less virulent strain causes variola minor.
The Orthopoxvirus genus also includes the vaccinia, monkeypox, cowpox, camelpox and ectromelia viruses; these agents resemble each other in electron micrographs and in many culture systems. They are also serologically cross–reactive. Monkeypox, which usually found in Africa, closely resembles smallpox; in monkeypox infections, the mortality rate is lower and the cervical and inguinal lymph nodes are highly enlarged.
Iowa State University
Iowa State University Center for Food Security and Public Health, "Smallpox" (2004). Center for Food Security and Public Health Technical Factsheets. 123.