People unknowingly drank contaminated water from the Thames River, which caused them to contract cholera. With each evacuation, the organism passed out of the body and, importantly, flowed into England’s water supply. This microorganism entered the body through food and drink, flowed through the alimentary canal where it multiplied and generated a poison that caused the body to expel water. Snow developed a novel theory about cholera in which the active agent was not an inanimate particle but was rather a living organism. Faced with the theory’s failure to explain cholera, he did what good scientists do-he changed his mind and began look for a new explanation. He went so far as to cover the sick with burlap bags, for instance, but the disease still spread. Originally, Snow-like everyone-accepted the miasma theory and tried many ingenious approaches based on the theory to block these airborne poisons from reaching other people. John Snow worked in London during these epidemics. But tried and true methods like quarantining the sick were strangely ineffective at slowing down this plague. Treatments, therefore, tended to be designed to stop poisonous dirt from spreading through the air. These particles were thought to be inanimate, and because microscopes at that time had incredibly poor resolution, it would be years before microorganisms would be seen. The majority medical opinion about cholera transmission at that time was miasma, which said diseases were spread by microscopic poisonous particles that infected people by floating through the air.
Doctors could not help the victims because they were mistaken about the mechanism that caused cholera to spread between people. Snow, a physician, watched as tens of thousands suffered and died from a mysterious plague. There were three main epidemics that hit London, and like a tornado, they cut a path of devastation through the city. In the nineteenth century, it was usually fatal. This is the story of how John Snow convinced the world that cholera was transmitted by water, not air, using an ingenious natural experiment ( Snow 1855).Ĭholera is a vicious disease that attacks victims suddenly, with acute symptoms such as vomiting and diarrhea. And one of the most interesting natural experiments was also one of the first difference-in-differences designs. All good difference-in-differences designs are based on some kind of natural experiment. When thinking about situations in which a difference-in-differences design can be used, one usually tries to find an instance where a consequential treatment was given to some people or units but denied to others “haphazardly.” This is sometimes called a “natural experiment” because it is based on naturally occurring variation in some treatment variable that affects only some units over time.