Key Terms You Must Know

📖 Key Terms You Must Know

Germ theory

The scientific understanding that specific microorganisms (bacteria and other pathogens) cause specific diseases. Developed by Louis Pasteur from 1861, and proved for specific diseases by Robert Koch from 1876. This replaced miasma theory as the dominant explanation for disease and transformed medicine, surgery, and public health. For the AQA exam, you must be able to explain not just what germ theory is, but WHY it was such a turning point: before it, doctors couldn't target the real cause of disease; after it, every advance in medicine became possible.

Miasma theory

The belief, dominant for over 2,000 years, that disease was caused by "bad air" — foul-smelling vapours rising from rotting organic matter. It was wrong, but it was not entirely useless: miasma-based reasoning led people to clean up sewage and rotting material, which happened to reduce disease even though the reasoning was incorrect. The crucial limitation was that miasma theory gave no route to identifying or killing specific disease agents. Once germ theory replaced it, targeted treatments became possible for the first time.

Koch's Postulates

A four-step scientific method developed by Robert Koch to prove that a specific microorganism causes a specific disease. Steps: (1) isolate the organism from a diseased animal; (2) grow it in a pure culture outside the body; (3) inject it into a healthy animal and show it produces the same disease; (4) re-isolate the same organism from the newly diseased animal. This method gave germ theory rigorous scientific proof and transformed medicine into an experimental science.

Spontaneous generation

The old theory (disproved by Pasteur's swan-neck flask experiment) that microorganisms appeared from nowhere — that rotting meat "generated" maggots and that germs arose spontaneously from decaying matter. Pasteur showed that if air was kept free of dust and microorganisms (using his specially shaped flask), decay did not occur — proving that germs came from outside, not from spontaneous generation.

Pasteurisation

The process of heating liquids to a specific temperature to kill harmful microorganisms, developed by Pasteur in the 1860s. Originally applied to wine and beer to prevent spoilage, it is now used for milk. Its significance for this topic: pasteurisation was a direct practical application of germ theory — the first time the knowledge that germs cause decay was turned into a practical technique. It established the principle that killing germs prevents disease, which underpins all antiseptic and antibiotic medicine.

Staining techniques

Methods developed by Koch and his team to make bacteria visible under a microscope by treating them with coloured dyes. Different bacteria absorbed different dyes, making them identifiable. Without staining, most bacteria are nearly transparent and very difficult to see even with a powerful microscope. Koch's staining techniques were a critical piece of laboratory technology that made the systematic identification of specific bacteria possible.