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Before the pioneers of microbiology proved otherwise, many infections were believed to be due to draughts, exposure to damp or cold – perpetuated in the term ‘catching a cold’ – or a miasma emanating from swamps (malaria). It was the perfection of better optical lenses for microscopes that led to the identification of bacteria and other microscopic organisms as the cause of these infectious diseases. Identification of viruses came later with invention of the electron microscope.

The 17th–18th-century German physician Robert Koch, who identified the tubercle bacillus and proposed that it was the cause of tuberculosis, was disbelieved by many of his contemporaries. In order to prove his hypothesis Koch established three most important elements of proof of causation of disease by a toxic agent.

Now known as Koch’s postulates, these are:

1 the proposed cause of the disease must always be shown to be present in a person suffering the disease;

2 removing the proposed cause cures the disease; and

3 reintroducing the putative cause re-establishes the condition.

Koch went on to fulfil these three requirements for proof in experimental animals. As a result of this evidence his hypothesis became a fact. ‘The Germ Theory of Disease’, to which Koch’s and others microbiologists’ work had given rise, was espoused by many practitioners as though it applied to all diseases whether caused by micro-organisms or not. Clearly, it is going to be difficult, and often ethically improper, to try to satisfy all three of Koch’s postulates in all cases, but the more that are met, the more likely the proposed causal mechanism is to be correct. A recent example to hit the headlines, but one that fulfilled none of Koch’s postulates, was the suggestion that the measles virus is a cause of autism.