ANTONIE VAN LEEUWENHOEK, a 17th-century Dutch businessman and scientist, was inordinately proud of his clean teeth. Every morning he scrubbed them with salt before rinsing his mouth with water. After eating, he carefully cleaned his teeth with a toothpick. Few people his age, he remarked in a letter in 1683 (when he was 50), had such clean and white teeth. Yet when he looked closely, he found “there remains or grows between some of the molars and teeth a little white matter”—now called dental plaque.
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As an expert microscopist who had observed tiny organisms in water a few years earlier, van Leeuwenhoek wondered whether they might also be present in this white matter. A microscope showed that it did indeed contain “many very small living animals, which moved very prettily”. His drawings of them, which he sent to the Royal Society in London, are considered the first definitive evidence of bacteria.
Few people suspected that such micro-organisms might cause disease. At the time, doctors followed the doctrine of Hippocrates, believing disease was caused by an imbalance of the “humours” within the body (blood, phlegm, yellow bile and black bile). Epidemic diseases, meanwhile, were attributed to miasma, the “bad air” given off by swamps or decomposing matter. Suggestions that disease might be transmitted by tiny living things were rejected by doctors. But the advent of the microscope showed these tiny creatures existed. Robert Hooke, an English scientist, published depictions of mucor, a microbial fungus, in the 1660s, and van Leeuwenhoek spotted what are now called protozoa and bacteria. Could the idea that tiny organisms caused disease have caught on in the late 17th century?
This notion, now known as germ theory, was only embraced in the second half of the 19th century. In the 1840s Ignaz Semmelweis, a Hungarian doctor, realised the importance of hand-washing and sterilisation of surgical instruments, but was ignored. In the 1850s John Snow traced cholera deaths in London to a neighbourhood water pump. Louis Pasteur demonstrated in the 1860s that fermentation and putrefaction depended on living micro-organisms that could be killed by heating. Joseph Lister, a British surgeon, then convincingly showed that using antiseptics to sterilise surgical instruments and clean wounds saved lives.