Many decisions are made in the face of risk and uncertainty and under circumstances where each person's individual decision affects other people's outcomes. The 2009 outbreak of swine flu (influenza A H1N1) provides a chance to study how lay people's perceptions of risks change over time and how those perceptions drive willingness to engage in precautionary behaviors (such as anti-viral medication use or self quarantine) that not only have consequences for the person who engages in the behaviors but that also affect the risks for others in the population. The investigators use an internet survey regarding people's risk perception and willingness to take precautionary measures to query cohorts of US adults starting a few days after the first news of the outbreak and continuing at regular intervals throughout the epidemic. The research examines the relationships over time among information from the media about the influenza outbreak, perceptions of risk, and interest in taking precautionary measures. This outbreak of a new infectious disease represents a rare opportunity to study how risk perceptions and precautionary behaviors change over time in response to information about how the hazard unfolds. The results could have implications not only for public health responses to natural disasters but also for an understanding of the basic mechanisms underlying risk perception and decision making under uncertainty.