Germ Theory: Pasteur and Koch

The development of germ theory in the second half of the 19th century was one of the most important turning points in the history of medicine. For the first time, scientists proved that diseases were caused by microorganisms (germs), overturning centuries of belief in miasma and the humours.

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The Problem: What Causes Disease?

Before germ theory, the dominant explanations for disease were:

Theory	Explanation
Miasma	Disease was caused by "bad air" arising from rotting organic matter
Spontaneous generation	Germs appeared as a result of disease, not as a cause
The Four Humours	Disease was caused by an imbalance of bodily fluids
God's punishment	Illness was divine retribution for sinful behaviour

Key Term: Spontaneous generation --- the widely held belief that microorganisms arose spontaneously from decaying matter, rather than being the cause of decay. Pasteur disproved this theory.

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Louis Pasteur (1822--1895)

Pasteur was a French chemist who made the critical breakthrough in understanding the role of germs in causing disease.

Key Discoveries