Cyprinidherpes virus 3(CyHV3) is the causative agent of alethal disease in common (Cyprinus carpio carpio) and Koi carp (Cyprnius carpio koi). It was discovered in the late 1990s and has rapidly spread worldwide among cultured common carp and ornamental koi. Previously known as koi herpesvirus, it has caused severe economic losses in the global carp industry with its spread being attributed to international trade. The virus is a member of the order Herepesvirales and family Alloherpesviridae. It has a linear, double stranded genome of approximately 295 kb in length consisting of a large central portion flanked by two 22 kb repeat regions. The genome encodes 156 open reading frames (ORFs) including 8 ORFs encoded by the repeat regions. The genome is packaged in an icosahedral capsid that is contained within viral glycoproteins and then a host derived lipid envelope, giving an overall virion of 170-200nm in diameter. At present, common and koi carp are the only species known to be affected by the virus.The viral particles are transmitted through faeces, sloughing of mucous and inflammatory cells, and secretions that are released into the water. The skin pores are the main source of entry and site of replication but the disease also spreads to the organs,particularly the kidneys.The viral particles are further spread when the carp come into contact with each other during grazing,spawning or when uninfected fish pick at the skin lesions of dead infected fish. It has also been reported that sediments act as a natural reservoir for the virus in freshwater. No evidence of vertical transmission of the disease has been reported. The disease only manifests at water temperatures between 18 °C and 28 °C but can remain latent until conditions are conducive to propagation. The incubation period for CyHv3 is usually 2-3 days. Clinical signs of infection include loss of appetite, sunken eyes, lethargy, gill necrosis and hyperaemia. The gills hypersecrete mucus, suffocating the fish and they tend to remain at the bottom of the pond with the dorsal fin folded. Some fish may experience loss of equilibrium and disorientation in the final stages of the disease but they may also show signs of hyperactivity. Mortality occurs within 6 to 22 days post infection and the disease is lethal in 80 to 100% of infected fish.