We examine the impact of influenza on stock markets. For the United States, a higher incidence of flu is associated with decreased trading, decreased volatility, decreased returns, and higher bid-ask spreads. Consistent with the flu affecting institutional investors and market makers, the decrease in trading activity and volatility is primarily driven by the incidence of influenza in the greater New York City area. However, the effect of the flu on bid-ask spreads and returns is related to the incidence of flu nationally. International data confirm our findings of a decrease in trading activity and returns when flu incidence is high.