<P> This increased severity has been attributed to the circumstances of the First World War . In civilian life, natural selection favors 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> <P> The fact that most of those who recovered from first - wave infections were now immune showed that it must have been the same strain of flu . This was most dramatically illustrated in Copenhagen, which escaped with a combined mortality rate of just 0.29% (0.02% in the first wave and 0.27% in the second wave) because of exposure to the less - lethal first wave . On the rest of the population it was far more deadly now; the most vulnerable people were those like the soldiers in the trenches--young previously healthy adults . </P> <P> Even in areas where mortality was low, so many were incapacitated that much of everyday life was hampered . Some communities closed all stores or required customers to leave orders outside . There were reports that health - care workers could not tend the sick nor the gravediggers bury the dead because they too were ill . Mass graves were dug by steam shovel and bodies buried without coffins in many places . </P> <P> Several Pacific island territories were particularly hard - hit . The pandemic reached them from New Zealand, which was too slow to implement measures to prevent ships carrying the flu from leaving its ports . From New Zealand, the flu reached Tonga (killing 8% of the population), Nauru (16%) and Fiji (5%, 9,000 people). </P>

Where did the flu pandemic of 1918 start