alt Hacker NewsRegarding the Spanish (or Kansas) flu, there is some evidence suggesting that the second wave was much deadlier than the first because of an unusual practice connected to World War I:
Soldiers infected with more virulent strains were more likely to be shipped to military hospitals, while those infected with less virulent strains were more likely to remain in the trenches.
The military hospitals were much more active vectors of transmission than the bays of the trenches, so the normal pattern of transmission was inverted, with the more virulent strains spreading faster than the less virulent ones.
Under normal conditions, the very sick would stay home while the less sick would go to work, which would tend to push highly virulent viruses toward becoming less virulent over time.
This reminds me of the fact that hospital-acquired pneumonia is the leading cause of death in ICUs in the US.
Among people who are already going to die, the thing that kills them the most is an opportunistic bacterial pneumonia that barely even exists outside of hospitals, or other compromised patients.