The doctrine that certain diseases are caused by specific micro-organisms. What it displaced was the two-thousand-year reign of miasma, the view that disease arose from the foul air of putrefaction. The turn was composed of several independent lines: in 1847 Semmelweis at Vienna cut mortality from puerperal fever from about 18 per cent to under 2 by handwashing; in 1854 Snow traced cholera to drinking water by mapping cases around the Broad Street pump in London; in the 1860s Pasteur refuted spontaneous generation with his swan-neck flasks and showed fermentation and putrefaction to be the work of micro-organisms; and between 1876 and 1884 Koch isolated the organisms of anthrax, tuberculosis, and cholera and set out four rules for establishing causation. The practical consequences were immense—surgical antisepsis, water treatment, quarantine, and vaccine development all rest on it. Yet variolation and vaccination had been working for a century before the theory arrived: practice preceding explanation is the rule in medical history, not the exception.
Gentlemen, it is we ourselves who carry death to the mothers.
—— Semmelweis to his Viennese colleagues, as reported in his Aetiology, Concept and Prophylaxis of Childbed Fever (1861)
History
In 1847 Ignaz Semmelweis, at the Vienna General Hospital, observed that the first obstetric clinic, staffed by physicians and medical students, had a far higher mortality from puerperal fever than the second, staffed by midwives. When a colleague cut himself during an autopsy and died with the same symptoms, Semmelweis concluded that physicians carried cadaveric particles on their hands from the dissecting room to the delivery room. He required washing in chlorinated lime, and mortality in the first clinic fell from about 18 per cent to under 2. The conclusion was right, and the cost was his colleagues' hostility—it amounted to charging physicians with killing their patients, and he could offer no mechanism while insisting in the strongest terms. He lost his post and died in an asylum in 1865. During the London cholera epidemic of 1854, John Snow plotted cases house by house and found them clustered within the reach of the Broad Street pump; more decisively, he exploited a natural experiment in which two water companies supplied alternate houses in the same streets, and compared cholera mortality among their customers. Miasma remained the majority view, and Snow likewise failed to convince most of his profession. What established the theory was the laboratory. In the 1860s Pasteur's swan-neck flasks showed that broth kept from airborne particles did not putrefy, refuting spontaneous generation; Lister then brought carbolic acid into surgery and post-operative infection fell sharply. From 1876 Koch, working in the poor conditions of a country practice, isolated the anthrax bacillus, then the tubercle bacillus (1882) and the cholera vibrio (1883), and set out four rules: the organism must be found in the diseased, cultured in pure form, produce the disease when inoculated into a healthy animal, and be recovered from that animal. With this, the correspondence between one disease and one micro-organism could for the first time be verified case by case.
Connections
Causes2
- VaccinationinformsThe practice worked roughly a century before the mechanism was known: it posed the theory's question rather than applying it, and Pasteur named his attenuated preparations after Jenner's
- The Black DeathinspiresIn 1894 Yersin isolated the plague bacillus, explaining the Black Death in the terms of bacteriology
Sources
- Charles E. Rosenberg, The Cholera Years: The United States in 1832, 1849, and 1866
- Steven Johnson, The Ghost Map: The Story of London's Most Terrifying Epidemic
- Ignaz Semmelweis (1861), Die Aetiologie, der Begriff und die Prophylaxis des Kindbettfiebers
- Thomas D. Brock, Robert Koch: A Life in Medicine and Bacteriology
- Germ theory of disease
Open questionswell attested
- The falls in puerperal fever mortality reported by Semmelweis varied considerably between clinics and years; the drop "from about 18% to below 2%" is the best-known set of figures from his First Obstetric Clinic, not an average for the whole hospital.
- Snow's Broad Street pump study is often told as "removing the pump handle ended the outbreak", but the epidemic had already passed its peak before the handle was removed; his main contribution was the map of cases and the natural experiment comparing customers of the Southwark and Vauxhall and the Lambeth water companies.
- To call miasma theory "completely wrong" overstates the case; it was wrong about the mechanism, but the urban cleansing, drainage and ventilation it inspired did reduce the burden of infectious disease.
- Koch's postulates had exceptions from the start (asymptomatic carriers, pathogens that cannot be cultured) and are used today in revised form, so they should not be treated as a fixed criterion.
Why it matters
The germ theory is often told as science defeating superstition, but its more valuable side is why it took so long. Semmelweis had data, a control, and an immediate effect, and failed; Koch had photographs down a microscope, a reproducible isolation and culture, and an explicit criterion, and succeeded. The difference lies not in the strength of the evidence but in its form: a statistical association asks people to believe in a mechanism they cannot see, while a colony on a plate lets them see it. This is not to say the profession was unreasonable, but that what a discipline accepts as evidence has itself a history: mid-nineteenth-century medicine had not yet taken statistics for one of its own instruments, and Semmelweis's numbers were to his colleagues not evidence but coincidence. A second layer is owed in fairness to miasma. Miasma was wrong in mechanism, and the cleansing of cities, the building of sewers, ventilation, and housing reform that it drove were considerably effective—Bazalgette's London sewers were built to abolish a stench and cut the waterborne path of cholera instead. A false theory may guide effective action, and a true one may for a long time find no purchase; to read effective as true, or false as useless, is to misread a good deal of medical history. A third layer concerns technology and science. Inoculation worked for roughly a century before the germ theory existed, practice preceding explanation; technology is not always the application of science, and is often the source of its questions.