Smallpox virus particles. Coloured transmission electron micrograph (TEM) of variola virus particles, the cause of smallpox. Variola is the most virulent member of the orthopoxvirus group. It is specific to humans, having no other animal hosts. Infection with variola causes a high temperature and skin spots that develop into scarring pustules. It is transmitted by respiratory droplets or by the pus. Smallpox was eradicated in the 1970's by a global vaccination programme. However, isolated cultures of the virus are still kept in laboratories for research purposes.