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Spanish Flu and the SIR model

Spanish Flu is a particularly deadly wave of influenza following the end of world war 2. Infecting 500 million people across the world and killing 50 to 100 million, the Spanish Flu is one of the deadliest natural disasters in human history. The total number of death by Spanish flu is greater than that of WW1.  Rather than killing the weak, elderly and infirm like normal influenzas do, the Spanish flu is most lethal against the strong and healthy adults. The New Yorker article traces the spread of the Spanish Flu and examine why it is so deadly.

In the SIR model, we defined a basic reproductive number R=pk where p is the probability of a flu spreading to others and k is the number of the people the host interact with. If R is greater than one, then the epidemics will spread across the network, if R is smaller than 1, then the epidemic is likely to be contained.

In the case of the Spanish Flu, the probability of infection p is very high, because many soldiers are weakened by malnourishment, shell shock, fatigue and chemical attacks. However, what made this epidemics particular deadly is the factors that enlarged K.

Before WW1, communities across the globe are largely contained, therefore, deadly diseases can wipe out the entire population but have a limited impact on the world. In ww1, the close quarters and frequent movements of the troops both increased transmission and mutation. Furthermore, the modern transportation enabled the disease to travel to even the remotest islands in the Pacific. Too add fuel to the fire, military censorship kept the news of large infections at bay, preventing the public from being aware of the disease and take preventive measures accordingly.

Interestingly, the circumstances of WW1 made every infected node to come into contact with a larger number of people. “In civilian life, natural selection favours 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.”(Wikipedia)


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