Typical influenzas kill weak individuals, such as infants (aged 0–2 years), the elderly, and the immunocompromised. Older adults may have had some immunity from the earlier Russian flu pandemic of 1889. Another oddity was that the outbreak was widespread in the summer and autumn (in the Northern Hemisphere); influenza is usually worse in winter.

In fast-progressing cases, mortality was primarily from pneumonia, by virus-induced pulmonary consolidation. Slower-progressing cases featured secondary bacterial pneumonias, and there may have been neural involvement that led to mental disorders in some cases. Some deaths resulted from malnourishment and even animal attacks in overwhelmed communities.

Figure 2. Three pandemic waves: weekly combined influenza and pneumonia mortality, United Kingdom, 1918–1919.

The second wave of the 1918 pandemic was much deadlier than the first (see figure 2). 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. This has been attributed to the circumstances of the First World War. In civilian life evolutionary pressures favour a mild strain: those who get really sick stay home, and those mildly ill continue with their lives, go to work and go shopping, preferentially spreading the mild strain. In the trenches the evolutionary pressures were reversed: soldiers with a mild strain remained where they were, while the severely ill were sent on crowded trains to crowded field hospitals, spreading the deadlier virus.

So the second wave began and the flu quickly spread around the world again. It was the same flu, in that most of those who recovered from first-wave infections were immune, but it was now far more deadly, and the most vulnerable people were those who were like the soldiers in the trenches—young, otherwise healthy adults. Consequently, during modern pandemics, health officials pay attention when the virus reaches places with social upheaval, looking for deadlier strains of the virus.