Three P. marinus life stages were found effective in transmitting the disease. P. marinus susceptibility and disease progression were positively correlated with temperature, salinity, and number of infective cells the oyster encountered. Temperature appeared to be the most important factor. The interaction between either temperature and salinity or between temperature and P. marinus dose significantly intensified the disease. The Pacific oyster, Crassostrea gigas, was less susceptible, but not completely resistant, to P. marinus compared to the eastern oyster, C. virginica.