In 1854, London was the largest metropolis in the world, and it was struck with a severe cholera outbreak. Many neighborhoods were overcrowded, the city wasn't prepared, and the hygienic conditions were very poor. This was the price to pay for the Industrial Revolution. Soho wasn't the gay-friendly neighborhood full of nightlife, pubs and sex shops that we all know today, but an overcrowded hell in which cesspools were constantly overrunning. So, the government decided to dump the waste into the River Thames. The result was a health disaster, but the cause was not understood at the time. It was the contaminated water that spread the cholera, but the thesis advanced by London physicians - in line with the science of the time - was that the spread of the disease was being caused by 'miasma', some kind of air-borne poisonous exhalations. Therefore, the citizens continued to drink the contaminated water, and became increasingly ill.

On August 31, right in Soho, the most violent cholera epidemic broke out. Within three days, 127 people had died. People were scared and began to run away. In the end, the death toll stood at 616 Sohoites. 'The most terrible outbreak of cholera which ever occurred in this kingdom,' wrote Dr John Snow, a 40-year-old physician from York. He is the one who managed to solve the mystery of the cholera epidemic in Soho.

Snow wasn't too popular among his colleagues, because he didn't believe in the miasma theory. To solve the mystery of the epidemic, he used an investigative method as if he also was a detective and a journalist: he collected data and analyzed the entire neighborhood. He interviewed many Sohoites and understands that there was something wrong with the public water pump in Broad Street: he was told that the taste of that water had changed just before the outbreak. He persuaded the authorities to remove the pump. However, he was still missing a piece of the puzzle: Infectious diseases are caused by microorganisms, but no one knew that yet. It was 1854, and Louis Pasteur was dealing for the first time with the destruction of microbes sometimes present in alcoholic beverages.

The York doctor had identified the source of the contagion, he sensed that that water had somehow brought on cholera, but he lacked the theoretical knowledge to justify his intuitions.  At the origin of the epidemic, he explained, there were the dirty diapers of a sick child, which were being thrown into a cesspool, which had contaminated the water of Broad Street. The authorities do not take it well. They censor his thesis, because it's too harsh to believe that those people had died because they had drunk water that had been contaminated with feces. They put the pump back, and Snow's work, On the Mode of Communication of Cholera, was torn to shreds by all medical journals.  When the epidemic ended, medical authorities explained that it had been caused “by overcrowding, poor sanitation, and poor ventilation.”

The Metropolitan Commission of Sewers also wanted more details about what had happened, and sent engineer Edward Cooper to inquire whether the sewer works that had been carried out in the neighborhood at that time had somehow released "noxious gases" from  an ancient pit where many bodies had been buried during the plague of 1665. And what did Cooper do? He brought along a map of Soho and marked the locations of the homes of those who had died.

When the officials of St James’s parish asked John Snow to conduct an investigation, he used Cooper's map, and changed it a little. First, he divided Soho into different areas, according to the proximity of the houses to the different water pumps - he is probably the first to use what we know today as the Voronoi diagram. Them, he enriched the map with graphs to support his thesis: the contagion has started from the water pump on Broad Street. Finally, he counted, one by one, all the deaths marking on the map where the deceased lived.  The highest concentration, of course, was near the infected source. “I found that nearly all the deaths had taken place within a short distance of the pump. There were only ten deaths in houses situated decidedly nearer to another street pump. In five of these cases the families of the deceased persons informed me that they always sent to the pump in Broad Street […]. In three other cases, the deceased were children who went to school near the pump in Broad Street.”

Snow supported his theory by linking water consumption to the spread of the disease. He presented a graph showing how, when the pump had been removed, the cases had decreased significantly. The English doctor's map wasn't simply a map, but a detailed statistical analysis.

John Snow died four years later, and his map was only reconsidered at the end of the 19th century. Today it is considered revolutionary, not only in the epidemiology sector, because it has changed the way we visualize data and information. The map was not used by Dr. Snow to get to the truth, but to explain it better and more effectively. The maps that we analyze every day to learn about the spread of the Coronavirus and the new cases of infection are, in some way, the descendants of that "augmented" map of Soho. The greatest example is "How the virus got out", the article with animated infographics that the interactive team of the New York Times has published in order to explain how the virus has spread from Wuhan to the rest of the world. But, there is a but: unlike the one drawn by John Snow, these maps tell us a lot, they tell a great story that includes other smaller and very painful ones, and they're great for in-forming, but they can't explain what the source of the contagion was - whether it be figurative or real, like the one on Broad Street.