i found a little bit of an explanation about virus mutations on a wikipedia article on the 1918 spanish flu:
"Deadly second wave
The second wave of the 1918 pandemic was much deadlier than the first. During the first wave, which began in early March, the epidemic resembled typical flu epidemics. Those at the most risk were the sick and elderly, and 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 more deadly form. This has been attributed to the circumstances of the first World War. In civilian life evolutionary pressures favor a mild strain: those who get really sick stay home, but 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 flu quickly spread around the world again. It was the same flu, in that those who recovered from first-wave infections were immune, but it was far more deadly, and the most vulnerable people were those 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."