Members of the family Herpesviridae have enveloped, spherical virions with characteristic complex structures consisting of symmetrical and non-symmetrical components (Table 1.Herpesviridae). The linear, double-stranded DNA genomes of 125–241 kbp contain 70–170 genes, of which 43 have been inherited from an ancestral herpesvirus. Herpesviruses have generally coevolved with their hosts and are highly adapted to them, and are likely to be associated with most mammalian, avian and reptilian species. Following primary infection, they are able to establish life-long latent infection, during which there is limited viral gene expression. Severe disease is usually observed only in the foetus, the very young, the immunocompromised or following infection of an alternative host.