The H7N7 epidemic in the Netherlands in 2003 affected a total of 255 commercial flocks in two distinct geographical and temporal clusters. The outbreak was situated in the Gelderse Vallei, the densest poultry production area in the Netherlands, in which over 10 million birds are kept in 984 flocks with a density of 4 flocks/ km 2 [12, 13] . Two months into the outbreak the infection passed to Limburg, another very dense poultry production area, where it continued to spread.