Ostreid herpesvirus-1 microvariant (OsHV-1) is a member of the family Malacoherpesviridae within the order Herpesvirales. OsHV-1 microvariants have been responsible for mass mortality events in commercially produced Pacific oysters (Crassostrea gigas) in Australia, New Zealand and Europe, since their first detection in France and Australasia in 2008 and 2010, respectively. OsHV-1 presents a significant threat to the Australian C. gigas industry (worth AU$53 million in 2007/08), with all age and size classes of C. gigas affected by the virus. Oysters <1 year of age are the most susceptible age class, with mortalities of 60% to 100% observed in France and Australia. Of great concern to the industry is the propensity of the virus to persist in a waterway after initial infection, resulting in recurrent, seasonal outbreaks of disease. The commercial production of C. gigas has ceased entirely in two key estuaries in NSW, Australia as a result of the recurrent disease caused by OsHV-1. The need to develop robust farm management strategies that will allow growers to produce C. gigas in the presence of OsHV-1 is apparent. In order to achieve this goal, a thorough understanding of the transmission and spread of OsHV-1, and the factors that lead to transmission and expression of the disease is required. One of the largest knowledge gaps concerns the transmission of the virus in the estuarine environment. This thesis was focused on addressing several of these gaps by assessing the transmission, detection and distribution of OsHV-1 in seawater, the occurrence of OsHV-1 disease outbreaks to identify periods of high and low risk, the persistence of OsHV-1 in farmed and wild oysters and the potential for them to act as sources of virus in subsequent outbreaks, and possible risk factors for OsHV-1 transmission and expression of clinical disease.