Louis Pasteur (1822-1895)
The French chemist Louis Pasteur developed germ theory, which became central to our understanding of disease. Using experiments and microscopes, Pasteur found that liquids such as beer and milk went off because of the rapid multiplication of very small organisms - germs - in those liquids. He investigated further and found that many of these micro-organisms could be killed by heating the liquid: a preservation method now called ‘pasteurisation’.
Pasteur applied his explanation of decaying liquids to solid matter as well. He showed experimentally that the decay of meat was caused by microbes. The chemist argued that this could explain disease as well as decay, claiming that disease was caused by the multiplication of germs in the body. He investigated his theory using silkworms and went on to develop a new form of vaccination - by chance he discovered that germs which had been weakened by long exposure to the air caused immunity to cholera in chickens. Pasteur’s work made a significant contribution to the development of the first ‘magic bullets’, chemicals developed to attack specific germs.