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This year marks the 100th anniversary of the deadliest event in human history. In 1918-1919, pandemic influenza appeared nearly simultaneously around the globe and caused extraordinary mortality (an estimated 50-100 million deaths) associated with unexpected clinical and epidemiological features. The descendants of the 1918 virus remain today; as endemic influenza viruses, they cause significant mortality each year. Although the ability to predict influenza pandemics remains no better than it was a century ago, numerous scientific advances provide an important head start in limiting severe disease and death from both current and future influenza viruses: identification and substantial characterization of the natural history and pathogenesis of the 1918 causative virus itself, as well as hundreds of its viral descendants; development of moderately effective vaccines; improved diagnosis and treatment of influenza-associated pneumonia; and effective prevention and control measures. Remaining challenges include development of vaccines eliciting significantly broader protection (against antigenically different influenza viruses) that can prevent or significantly downregulate viral replication; more complete characterization of natural history and pathogenesis emphasizing the protective role of mucosal immunity; and biomarkers of impending influenza-associated pneumonia.

A century ago, nonpharmaceutical interventions such as school closings, restrictions on large gatherings, and isolation and quarantine were the centerpiece of the response to the Spanish Flu. Yet, even though its cause was unknown and the science of vaccine development was in its infancy, considerable enthusiasm also existed for using vaccines to prevent its spread. This desire far exceeded the scientific knowledge and technological capabilities of the time. Beginning in the early 1930s, however, advances in virology and influenza vaccine development reshaped the relative priority given to biomedical approaches in epidemic response over traditional public health activities. Today, the large-scale implementation of nonpharmaceutical interventions akin to the response to the Spanish Flu would face enormous legal, ethical, and political challenges, but the enthusiasm for vaccines and other biomedical interventions that was emerging in 1918 has flourished. The Spanish Flu functioned as an inflection point in the history of epidemic responses, a critical moment in the long transition from approaches dominated by traditional public health activities to those in which biomedical interventions are viewed as the most potent and promising tools in the epidemic response arsenal.

The 1918 influenza pandemic spread rapidly around the globe, leading to high mortality and social disruption. The countermeasures available to mitigate the pandemic were limited and relied on nonpharmaceutical interventions. Over the past 100 years, improvements in medical care, influenza vaccines, antiviral medications, community mitigation efforts, diagnosis, and communications have improved pandemic response. A number of gaps remain, including vaccines that are more rapidly manufactured, antiviral drugs that are more effective and available, and better respiratory protective devices.