The second wave of the 1918 pandemic was much more deadly than the first. The first wave had resembled typical flu epidemics; those most at risk were the sick and elderly, while younger, healthier people recovered easily. By August, when the second wave began in France, Sierra Leone, and the United States,[99] the virus had mutated to a much more deadly form. October 1918 was the month with the highest fatality rate of the whole pandemic.[100]