Until the mid-nineteenth century, decay, putrefaction, and many diseases were believed to arise by "spontaneous generation" (abiogenesis) — directly from lifeless matter. Meat seemed to breed maggots on its own, grape must to sour on its own. The French chemist Louis Pasteur (1822–1895) demolished this belief with a single elegant experiment.

He placed a nutrient broth in "swan-neck" flasks, their necks drawn out into an S-shape, and boiled it. Air could enter the flask freely; but dust and microbes in the air were caught in the curved neck and never reached the broth. The liquid stayed clear and unspoiled for months. When the neck was broken, or the liquid tipped to touch the bend, within hours it clouded and filled with microbes. The conclusion was decisive: life comes only from prior life; decay is the work of invisible organisms carried in the air.

Pasteur put the principle to work. He showed that the souring of wine and milk came from microbes, and developed a way to kill them by heating the liquid gently — what we now call pasteurization. Then he carried the same logic to disease: if invisible organisms can spoil milk, they can also sicken the body. This "germ theory" was reinforced by Robert Koch's work linking specific microbes to specific diseases.

The consequences snowballed. Joseph Lister lowered surgical deaths by applying antiseptics in the operating room; Pasteur himself developed vaccines for rabies and anthrax. The source of disease was no longer bad air or divine punishment but a concrete organism that could be confronted. The antibiotics, vaccines, and public-health infrastructure of the twentieth century were all built upon this conceptual break.