<P> A study--conducted by He et al.--used a mechanistic modelling approach to study the three waves of the 1918 influenza pandemic . They tried to study the factors that underlie variability in temporal patterns, and the patterns of mortality and morbidity . Their analysis suggests that temporal variations in transmission rate provide the best explanation and the variation in transmission required to generate these three waves is within biologically plausible values . </P> <P> Another study by He et al. used a simple epidemic model, to incorporate three factors including school opening and closing, temperature changes over the course of the outbreak, and human behavioral changes in response to the outbreak to infer the cause of the three waves of the 1918 influenza pandemic . Their modelling results showed that all three factors are important but human behavioral responses showed the largest effects . </P> <P> The second wave of the 1918 pandemic was much deadlier 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 . 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 . </P> <P> This increased severity has been attributed to the circumstances of the First World War . In civilian life, natural selection favours a mild strain . Those who get very ill stay home, and those mildly ill continue with their lives, preferentially spreading the mild strain . In the trenches, natural selection was reversed . Soldiers with a mild strain stayed where they were, while the severely ill were sent on crowded trains to crowded field hospitals, spreading the deadlier virus . The second wave began and the flu quickly spread around the world again . Consequently, during modern pandemics health officials pay attention when the virus reaches places with social upheaval (looking for deadlier strains of the virus). </P>

What american city was one of the hardest hit by the 1918 flu