After smallpox was declared eradicated by WHO in December 1979, vaccination programs were terminated. A re-emergence of the disease either by accident or as a result of biological warfare would meet an unprotected population and could result in an epidemic that could be difficult to control. Mass vaccination to combat a smallpox epidemic could be challenging because the only approved smallpox vaccine, Vaccinia Virus, can have severe side effects. Nevertheless, some governments stockpile Smallpox vaccine to insure against the possibility. However, the development of specific and effective antiviral drugs is prioritized. One possible approach would be to use the specificity of the thymidine kinase of poxvirus for the purpose, in a similar way that it is used for drugs against herpesvirus. One difficulty is that the poxvirus thymidine kinase belongs to the same family of thymidine kinases as the human thymidine kinases and thereby is more similar chemically. The structure of poxvirus thymidine kinases has therefore been determined to find potential antiviral drugs. The search has, however, not yet resulted in a usable antiviral drug against poxviruses.