The death toll from the 1918 influenza epidemic approached 650,000 Americans within the country, the greatest loss of life from any single epidemic in United States history. The second wave of the 1918 pandemic was much deadlier than the first. The first wave had resembled typical flu epidemic; those most at risk were the sick and elderly, while younger, healthier people recovered easily. But in August, when the second wave began in France, Sierra Leone and the United States, the virus had mutated to a much deadlier form. This has been attributed to the circumstances of the First World War.