Infectious diseases that can spread rapidly have always been able to cause outbreaks. Rapid international travel has allowed what were once localised public health issues to extend beyond national borders and develop into global pandemics. The challenges posed by a novel disease may be enormous, and include initial recognition in a previously unexposed population, development of a clinical case definition allowing accurate descriptive epidemiology of its progression in the community, laboratory diagnosis of a novel pathogen, establishment of evidence-based management protocols for patient care, identification of routes of transmission allowing targeted public health interventions, and development of therapeutic interventions to reduce transmission or prevent severe effects. The consequences of large-scale outbreaks extend beyond their direct impact on human health. They may have economic effects as a direct result of illness or of control measures, and impacts on social wellbeing, including damage to national and local infrastructure, civil disruption, or even conflict.