Proof that diseases are caused by germs invading the body from outside, and refutation of the idea that complex life could arise by ‘spontaneous generation’ from non-living things, came from the work of Louis Pasteur in France in the second half of the nineteenth century. In 1854, he became Professor of Chemistry at the University of Lille, where his duties included advising local industries on overcoming practical problems. The work for which he became famous stemmed from his work with the brewing industry, finding ways to prevent products going sour.
Pasteur studied sour beer under the microscope, and found that it was filled with a large number of tiny organisms. It had previously been thought that such contamination was produced by the beer going sour; but Pasteur was convinced that they were the cause of the beer going sour. He showed that fermentation is produced by naturally occurring yeasts, and that grape juice drawn by a hypodermic needle from inside the fruit would never ferment in a sterilized container, because the yeast was present only on the skin of the fruit. The sourness in wine and beer that had ‘gone off’ was being caused by contamination from outside, which triggered the production of lactic acid in the brew. The same problem occurred with milk. Pasteur found that heating milk to a temperature between 60 ºC and 100 ºC, and then cooling it, killed the offending organisms (bacteria), preventing it going sour. The process, tested in 1862, became known as Pasteurisation.
But this was just a beginning. Although Pasteur was not the first person to come up with the germ theory of disease, it was a minority view in the 1850s, vociferously opposed by the medical establishment. Pasteur promoted the idea and proposed that disease could be prevented by stopping microorganisms entering the body. This encouraged others to develop the use of antiseptics and promote cleanliness in surgery, leading to a dramatic decline in hospital death rates. In another series of experiments, Pasteur boiled broths in glass flasks to ‘Pasteurise’ them, then left them connected to the air through narrow tubes with filters to prevent any contamination getting in. Nothing grew in the broths. He wrote: ‘Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment. There is no known circumstance in which it can be confirmed that microscopic beings came into the world without germs, without parents similar to themselves.’
Pasteur went on to work with diseases, and discovered the technique of preparing a weakened form of a disease-causing agent to act as a vaccine. This was different from Edward Jenner’s pioneering work (see here), which had used a natural but weaker disease (cowpox) to provide immunity against a deadly disease (smallpox). In 1879 Pasteur discovered that after chickens were injected with a culture of chicken cholera that had accidentally been left to one side for a month they became unwell, but recovered. His assistant was going to throw away the ‘faulty’ culture, but Pasteur stopped him. He realized that the chickens might now be immune to the disease, and that proved to be the case. Pasteur went on to develop vaccines for anthrax and rabies in a similar fashion, weakening (or killing) the bacteria in various ways, and was responsible for suggesting that Jenner’s term ‘vaccine’ should be applied to all such artificially weakened disease organisms.
The work on rabies led Pasteur into a professional indiscretion that could have had serious consequences. The vaccine was developed by a medical doctor, Emile Roux, who was a colleague of Pasteur. It had been tested successfully on 50 dogs when, on 6 July 1885 a nine-year-old boy, Joseph Meister, was bitten by a rabid dog and brought to Pasteur. After discussing the situation with his team, and with time of the essence, Pasteur personally administered the vaccine to the boy: ‘The death of this child appearing to be inevitable, I decided, not without lively and sore anxiety, as may well be believed, to try upon Joseph Meister, the method which I had found constantly successful with dogs. Consequently, sixty hours after the bites, and in the presence of Drs Vulpian and Grancher, young Meister was inoculated under a fold of skin with half a syringeful of the spinal cord of a rabbit, which had died of rabies. It had been preserved (for) fifteen days in a flask of dry air. In the following days, fresh inoculations were made. I thus made thirteen inoculations. On the last days, I inoculated Joseph Meister with the most virulent virus of rabies.’23
Pasteur was not a qualified medical doctor, and would have been in serious trouble if things had gone wrong. Fortunately, however, the boy recovered, and the positive publicity helped to ensure the widespread acceptance of vaccination.