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The Rules of Contagion

Why Things Spread – and Why They Stop

By Adam Kucharski
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The Rules of Contagion by Adam Kucharski
Synopsis

The Rules of Contagion (2020) takes a scientific and mathematical look at how viruses spread, and how ideas, behavior and popular online content all follow similar patterns. By following the rules of contagion, we can gain insight into the spread of ideas, what causes financial disasters, and how harmful acts like gun violence can also infect a community.

Key idea 1 of 8

Mathematics and models forever changed the way we study contagious events.

Throughout human history, we’ve had to contend with outbreaks of contagious or infectious diseases. The good news is, many of these diseases – such as malaria, smallpox, and measles – have been greatly reduced. Thanks to advances in vaccines, hygiene, and our understanding of how disease moves, we’re much better at coping with the spread of disease than we were in the past.

One of the tools we’ve benefitted from is the use of scientific models. These have allowed us to analyze the spread of contagions and, in some cases, accurately predict our risk of danger.

The key message here is: Mathematics and models forever changed the way we study contagious events.

One of the pioneers in using math to study outbreaks was the nineteenth-century British surgeon Ronald Ross. In 1883, Ross was stationed in Bangalore, India. It was there that he first encountered a severe mosquito problem. But what was causing it? Ross was among the first to recognize that mosquito numbers were directly related to the presence of stagnant water.

Ten years later, on sabbatical in London, Ross gained some critical insight from fellow doctor Patrick Manson. After studying parasites in China, Manson was convinced that when mosquitoes fed on blood infested with parasites, they could also become carriers of those parasites. This was vital information for Ross. He saw it as a likely explanation for the spread of malaria, one of humanity's oldest infectious diseases. Could parasite-carrying mosquitoes be the culprit?

Ross proved his theory by getting mosquitoes to bite a bird infected with malaria. He showed how these mosquitoes could then transmit the disease to healthy birds. Ross then took his ideas a step further. He proposed ways of controlling malaria. For this, he made calculations and created models, publishing them in a 1910 book titled The Prevention of Malaria. For the first time, Ross explained how infection rates could be reduced and controlled by a reduction in the number of mosquitoes in an area.

For example, Ross’s data showed that it took around 48,000 mosquitoes to generate an average of one new human infection. So removing or treating stagnant water would have a direct influence on the number of new infections. He then laid out the importance of two statistics, the rate of infection and the rate of recovery, and showed that once the rate of recovery surpassed the rate of infection, the number of cases would eventually reach zero.

This marked a whole new way of looking at infectious disease.

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