A point made again and again in Johnson's narrative is how, despite the number of extremely bright and well intentioned people involved in trying to protect London from diseases, it took a combination of very particular circumstances as well as surprising levels of coordination and collaboration in order to break the back of this medical mystery. He also makes the point that nothing happened easily or quickly. Dr. John Snow made his case, but it was the better part of a decade before the theory of the waterborne nature of the disease effectively displaced the competing miasma theory (spread through the air.)
And so Snow's immunity to the miasma theory was as overdetermined as the theory itself. Partly it was an accident of professional interest; partly it was a reflection of his social consciousness; partly it was his consilient, polymath way of making sense of the world. He was brilliant, no doubt, but one needed only to look to William Farr to see how easily brilliant minds could could be drawn into error by orthodoxy and prejudice. Like all those ill-fated souls dying on Broad Street, Snow's insight lay at the intersection of a series of social and historical vectors. However brilliant Snow was, he would never have proved his theory - and might well have failed to concoct it in the first place - without the population densities of industrial London, or Farr's numerical rigor, or his own working-class up-bringing. This is how great intellectual breakthroughs usually happen in practice. It is rarely the isolated genius having a eureka moment in the lab. Nor is it merely a question of building on precedent, of standing on the shoulders of giants, in Newton's famous phrase. Great breakthroughs are closer to what happens in a flood plain: a dozen separate tributaries converge, and the rising waters lift the genius high enough that he or she can see around the conceptual obstructions of the age.