Epidemiology and pathology
Transmission and mutation
As U.S. troops deployed en masse for the
war effort in Europe, they carried the Spanish flu with them.
The
basic reproduction number of the virus was between 2 and 3.
[80] The close quarters and massive troop movements of
World War I hastened the pandemic, and probably both increased transmission and augmented mutation. The war may also have reduced people's resistance to the virus. Some speculate the soldiers' immune systems were weakened by malnourishment, as well as the stresses of combat and chemical attacks, increasing their susceptibility.
[81][82] A large factor in the worldwide occurrence of the flu was increased travel. Modern transportation systems made it easier for soldiers, sailors, and civilian travelers to spread the disease.
[citation needed] Another was lies and denial by governments, leaving the population ill-prepared to handle the outbreaks.
[83]
The severity of the second wave has been attributed to the circumstances of the First World War.
[84] 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 look for deadlier strains of a virus when it reaches places with social upheaval.[85] The fact that most of those who recovered from first-wave infections had become 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.[86] For the rest of the population, the second wave was far more deadly; the most vulnerable people were those like the soldiers in the trenches – adults who were young and fit.[87]
After the lethal second wave struck in late 1918, new cases dropped abruptly. In Philadelphia, for example, 4,597 people died in the week ending 16 October, but by 11 November, influenza had almost disappeared from the city. One explanation for the rapid decline in the lethality of the disease is that doctors became more effective in the prevention and treatment of pneumonia that developed after the victims had contracted the virus. However, John Barry stated in his 2004 book
The Great Influenza: The Epic Story of the Deadliest Plague In History that researchers have found no evidence to support this position.
[8] Another theory holds that the 1918 virus mutated extremely rapidly to a less lethal strain. Such
evolution of influenza is a common occurrence: there is a tendency for pathogenic viruses to become less lethal with time, as the hosts of more dangerous strains tend to die out.
[8] Some fatal cases did continue into March 1919, killing one player in the
1919 Stanley Cup Finals.
