The disease caused by koi herpesvirus (KHV) is amongst the most dramatic examples of an emerging disease of fish. KHV is a member of the genus Cyprinivirus in the family Alloherpesviridae. Koi herpesvirus disease is relatively host-specific; although other cyprinid species have been shown to be susceptible, only common carp (C. carpio) and its ornamental subspecies, the koi carp, have been involved in the explosive losses that have been reported globally in areas where the virus has been first introduced [47]. The enveloped virion of KHV, formally classified as the species Cyprinid herpesvirus 3, has a morphology typical of herpesviruses and contains a double-stranded DNA genome of approximately 295 kbp [5]. Molecular analysis has shown little variation among isolates, as might be expected for a virus that is being rapidly disseminated by the global movement of infected fish [37]; however, minor variation has been reported that may reflect at least two independent introductions or emergence events of KHV [68]. A significant problem is that once fish are infected, the virus persists for some period of time in a latent or carrier state without obvious clinical signs [125]. It appears that the movement of such carriers via the extensive trade in cultured ornamental fish has resulted in the rapid appearance of the disease in many regions of the world [41]. In addition, the release or stocking of ornamental fish into ponds and other natural waters has resulted in the introduction of KHV to naive wild populations, where the initial exposure can result in substantial mortality