Smallpox is a highly contagious and deadly disease, which was eradicated worldwide by 1980 after a global vaccination campaign, and no longer occurs naturally. Unique to humans, the disease is caused by the variola virus, and is transmitted from person to person through direct contact with contaminated fluids and objects, as well as through the air. Historically, about 30% of those who became infected with smallpox died from the illness.

Despite being eradicated, smallpox is considered by many governments to be a significant bioterrorism threat and as routine vaccination stopped in the 1970’s, the majority of the world’s population today are highly vulnerable. Re-emergence of smallpox could occur as result of deliberate or accidental release of samples that are kept for research purposes or may be synthesized in a lab.