The World Health Organization (WHO) is central to the international community's efforts to control infectious disease outbreaks. In recent years, however, the Organization's powers have undergone substantial revision following a series of interconnected global events including the 2003 severe acute respiratory syndrome outbreak, the revised International Health Regulations, the emergence and spread of avian influenza, and more recently, the 2009 H1N1 Swine Flu pandemic. This paper explores how the WHO's role, authority and autonomy have been shaped and re-shaped, and examines what this may mean for the future of global health security.