Rabbit haemorrhagic disease virus emerged in China in 1984, and has killed hundreds of millions of wild rabbits in Australia and Europe. In the UK there appears to be an endemic non-pathogenic strain, with high levels of seroprevalence being recorded, in the absence of associated mortality. Using a seasonal, age-structured model we examine the hypothesis that differences in rabbit population demography differentially affect the basic reproductive rates (R(0)) of the pathogenic and non-pathogenic strains, leading to each dominating in some populations and not others. The strain with the higher R(0) excluded the other, with the dynamics depending upon the ratio of the two R(0) values. When the non-pathogenic strain dominated, the pathogenic strain caused only transient mortality, although this could be significant when the two R(0) values were similar. When the pathogenic strain dominated, repeated epidemics led to host eradication. Seroprevalence data suggest that the non-pathogenic strain may be protecting some, but not all UK populations, with half being 'at risk' from invasion by the pathogenic strain and a fifth prone to significant transient mortality. We identify key questions for empirical research to test this prediction.