This is a really good account of the outbreak of cholera that led to John Snow’s famous map, showing that a particular water pump was the culprit. He traces the history of how London dealt with sewage and how it became such a big issue, and also examines some of the main characters in the drama of trying to stop the outbreak — and trying to challenge miasma theory, which was so much the paradigm at the time. There isn’t a lot of specific science stuff here, but Johnson makes clear why the cholera pathogen is so deadly in a very accessible way.
The only weird part is in the conclusion/afterword to the book, where Johnson talks more generally about the risks to city life and starts discussing nuclear war and terrorism. It seems very much a non-sequitur, and adds nothing to the book to my mind.