The influenza pandemic of 1918 killed more than 50 million people. Why was 1918 such an outlier? I. W. Brewer, a US Army physician at Camp Humphreys, Virginia, during the First World War, investigated several factors suspected of increasing the risk of severe flu: length of service in the army, race, dirty dishes, flies, dust, crowding, and weather. Overcrowding stood out, increasing the risk of flu 10-fold and the risk of flu complicated with pneumonia five-fold. Calculations made with Brewer's data show that the overall relationship between overcrowding and severe flu was highly significant (P < .001). Brewer's findings suggest that man-made conditions increased the severity of the pandemic flu illness.