To appropriately respond to emergent diseases, public health authorities must make a variety of decisions, under time and resource constraints. To this end, there have been information systems in the public health sector, for its decision support. Nevertheless, during the Swine-flu outbreak in Japan, the national surveillance system did not perform well enough, in the actual emergency situations. This paper presents the case of the 2009 pandemic-flu response in Japan, and clarifies the requirements for information systems in the public health sector, with respect to emergency management. The case study suggested the following lessons. First, information systems in public health domain must possess enough flexibility to accommodate additional surveys and features to dynamically modify the existing ones, to address exceptional situations. Secondly, virtualization technologies can be a solution for flexible management of system resources in emergency situations. Lastly, we found that the actual challenge lies in the number of parties involved, including public health authorities, medical institutions, care providers, and patients, which count up to millions, or billions, suggesting the needs for pervasive computing, as a reasonable course to take in the domain.