SUMMARY               The present methods allow the detection of viable, infectious DNA viruses in a sample.  In one embodiment, the present methods determine whether a DNA virus is present in a sample by contacting the sample with a first group of host cells capable of being infected by the DNA virus and transiently transfecting the host cells with a reporter construct.  The reporter construct comprises a reporter sequence operably linked to a virus-specific promoter sequence that enhances the transcription of the reporter sequence when the host cell is infected by the DNA virus to be detected.  The present method further includes the step of determining an expression level of the reporter sequence in the host cells, thereby determining whether the DNA virus is present in the sample.  Transient transfection can be accomplished by transduction, electroporation, heat shock, or lipofection. The present methods are particularly advantageous for detecting cytoplasmic-replicating DNA viruses, including those of the Poxviridae family such as vaccinia and variola (smallpox), in which case the promoter sequence can be the sequence of SEQ ID NO. 1.  The present methods can also be used to detect viruses of the Hepadnaviridae family, such as the hepatitis B virus.  Such viruses can be detected, e.g., in samples comprising tissue from a human or non-human animal subject, such as blood, plasma, cerebrospinal fluid, or saliva.  Alternatively, the sample can be derived from a subject treated with a therapeutic viral construct. In some embodiments, the virus-specific promoter used in the present methods enhances the transcription of the reporter sequence in the presence of a plurality of DNA viruses, generally viruses from the same family.  This embodiment is advantageous when a positive control assay is performed together with the present methods.  The positive control assay includes the steps of transiently transfecting a second group of host cells with the reporter construct, infecting the second group of host cells with a second DNA virus, and then determining an expression level of the reporter sequence in the second group of host cells, thereby determining that the presence of the DNA virus of interest in the sample can be detected.  In another embodiment, the present methods determine whether a poxvirus is present in a sample.  In this embodiment, host cells capable of being infected by a poxvirus are contacted with a sample and transiently transfected with a reporter construct.  The reporter construct comprises a reporter sequence operably linked to a poxvirus-specific promoter sequence.  The expression level of the reporter sequence in the host cells is then determined, thereby determining whether the poxvirus is present in the sample.  The promoter sequence used in this embodiment can be, for example, any of SEQ ID NO. 1, SEQ ID NO.2, SEQ ID NO. 3, or SEQ ID NO. 4.  This embodiment can further comprise the step of comparing the expression level of the reporter sequence to a calibration curve in order to quantitatively determine the amount of poxvirus in the sample, where the data points on the calibration curve are determined by contacting samples having a known titer of the poxvirus with respective groups of host cells capable of being infected by the poxvirus, transiently transfecting the host cells with a reporter construct comprising a reporter sequence operably linked to a poxvirus-specific promoter sequence, and determining an expression level of the reporter sequence in each of the groups of host cells.