Chapter I
Miasma
In the 1840s disease was widely blamed on miasma, foul air rising from rot and filth. In 1847 Ignaz Semmelweis, a doctor at Vienna's maternity hospital, noticed that mothers died of childbed fever far more often on the ward run by doctors, who came from the dissecting room, than on the midwives' ward. He ordered handwashing in chlorinated lime, and deaths fell roughly tenfold. His colleagues were offended by the suggestion that their hands carried death, and the practice lapsed after he left.
In 1854, in London, John Snow mapped cholera deaths in Soho and traced them to one water pump. He argued that cholera was carried by something in the water, not the air. Neither man could see the agent. Cell theory had established what microbes were, but not what they did.
Chapter II
Pasteur and Koch
Louis Pasteur, a chemist, came to microbes through fermentation, which he showed is the work of living yeast. In 1859–62 he settled an old dispute: does life arise spontaneously in broth? In his swan-neck flasks, boiled broth open to air but protected from falling dust stayed sterile indefinitely. Microbes come only from other microbes. The Glasgow surgeon Joseph Lister, reading Pasteur, reasoned that wound infections were caused by microbes and in 1867 began treating wounds with carbolic acid. Deaths after surgery fell sharply.
The proof that a particular microbe causes a particular disease came from Robert Koch, a German country doctor who worked at home with a microscope his wife gave him. In 1876 he traced the life cycle of the anthrax bacterium. His laboratory developed ways to grow bacteria in pure culture on solid media, using agar, which Fanny Hesse suggested from her kitchen, and in 1882 he identified the bacterium that causes tuberculosis. Pasteur and Koch, divided by the Franco-Prussian war, became bitter rivals, and their institutes raced to find the causes of cholera, diphtheria, plague and more. Within two decades the agents of most major bacterial diseases were known.
Chapter III
A Closer Look: Snow's Natural Experiment
In 1854 two water companies supplied the same districts of south London, their pipes running down the same streets. The Southwark and Vauxhall Company drew its water from the Thames at Battersea, downstream of London's sewage outfalls. The Lambeth Company had moved its intake upstream to Thames Ditton, above the sewage. Households had not chosen their company by health or wealth. As Snow put it, the two groups differed in nothing but their water.
Snow went door to door to find which company supplied each house where someone had died of cholera. For the first seven weeks of the epidemic:
| Water supply | Houses | Cholera deaths | Deaths per 10,000 houses |
|---|---|---|---|
| Southwark and Vauxhall | 40,046 | 1,263 | 315 |
| Lambeth | 26,107 | 98 | 37 |
For example, . Houses drinking the sewage-contaminated water suffered about eight and a half times the death rate of their neighbours. The comparison removed the other explanations that miasma theorists offered. The houses were in the same streets, breathed the same air and were as rich or poor as each other. Only the water differed.
This is the logic of a controlled comparison, applied to a population rather than a laboratory. Epidemiologists still use it, and randomised trials are designed to create it deliberately. Snow never saw the cholera bacterium. Filippo Pacini in Florence had described it in 1854, but his work was ignored, and Koch rediscovered it in 1884.
Chapter IV
Antibiotics
Vaccines and hygiene reduced infectious disease, but once infected, a patient had little defence. In 1928 Alexander Fleming noticed a mould on a culture plate killing the bacteria around it. He could not purify the substance, penicillin, in useful amounts. At Oxford in 1939–41, Howard Florey, Ernst Chain and Norman Heatley did, and showed it cured infections in mice and then people. By D-Day it was being mass-produced.
Antibiotics transformed medicine, and their overuse is now eroding them as resistant bacteria evolve. Meanwhile DNA sequencing has shown that the microbes grown in laboratories are a small minority of those that exist. How the body fights microbes by itself became immunology.