A key limit to sustainable salmon production is imposed by the need to control parasitic sea lice. Resistance to sea lice medicines is increasing, and so other methods of control are needed. One method that has proved successful and is increasingly applied in practice, is the use of cleaner fish such as wrasse that eat the lice off cohabiting salmon. Unfortunately, these wrasse can carry infections, some of which they can transmit to salmon. Recently, wrasse held on salmon farms in the Shetland Islands off north Scotland were found to be infected with viral haemorrhagic septicaemia virus (VHSV); VHS disease causes high mortality in trout and other fish. Although European VHSV has never been detected in salmon (in spite of intensive sampling), even a low probability risk may become significant under prolonged and intimate exposure of salmon of infected cohabiting wrasse, if this could lead to emergence of a salmon VHS with consequences similar to VHS in trout. However, the substantial benefitsof using wrasse for lice control must be weighted afgainst this risk. Therefore, the potential for disease emerging in salmon from wrasse, due to VHSV or other pathogens, has been analysed using a risk modelling framework. Wrasse could be a source of pathogens if: (1) infected on introduction to the salmon farm, (2) they become infected from wild reservoirs on farm, or (3) pathogens evolve virulence in the wrasse under farm conditions. Pathogens in wrasse it must then transmit to salmon. Even should such an emergence event occur, good biosecurity and area management practices could greatly limit its consequences. In Shetland, VHSV in wrasse originated from a wild reservoir in the region, but no transmission occured to salmon and the wrasse were culled. Risk associated with wrasse must be considered relative to other routes by which infection could reach the salmon farm, such as movements of salmon. We conclude disease risk from wrasse use is acceptable for salmon farming - provided controls are in place, including using hatchery reared (not wild caught) wrasse, surveillance of both wrasse and salmon for clinical disease, and area-management of salmon farms (including synchronised fallowing).