The classification of novel disease events is central to public health action surrounding them. Drawing upon the sociology of scientific classification, this article examines the role and contestation of the World Health Organization’s Pandemic Alert Phases, as applied to the spread of 2009/10 H1N1 Influenza. The analysis of World Health Organization texts, including policy documents, public statements and epidemiological documents, has been utilized to examine the Organization’s actions and public narratives around the event of H1N1. Analytically, the functional role of such classificatory schemes and the social construction of scientific classifications are examined. It is argued that in understanding the World Health Organization’s 2009/10 application of the Pandemic Alert Phases, the critical limitation of the functions served by the classificatory scheme led to the breakdown of its construction. This case study highlights the importance of classification for the successful production of scientific ‘facts’, the constructed nature of classificatory systems and the potential for contestation that arises when such classifications do not adequately fulfil their functional roles.