Emergence and reemergence of smallpox: the need for development of a new generation smallpox vaccine.

The review summarizes the archive data on smallpox, history of ancient civilizations, and the most recent data on the genome organization of orthopoxviruses, their evolutionary relationships, and the time points of smallpox emergence. The performed analysis provides the grounds for the hypothesis that smallpox could have emerged several times as a result of evolutionary changes in the zoonotic ancestor virus and disappeared due to insufficient population size of ancient civilizations. Smallpox reemerged in the Indian subcontinent approximately 2500-3000 years before present, which resulted in endemization of this anthroponotic infection, which had been preserved until the smallpox eradication in the 20th century AD. The conclusion suggests a potential possibility of future variola virus reemergence, presenting a great menace for mankind, as well as the need for development of new safe smallpox vaccines, design of anti-smallpox drugs, and activation of the control of zoonotic human orthopoxvirus infections.