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Germ Theory

The atmospheric germ theory, from a lecture given to the Royal College of Surgeons in Edinburgh, 1868.

The atmospheric germ theory, from a lecture given to the Royal College of Surgeons in Edinburgh, 1868.

Credits:Wellcome Library, London.

Germ theory states that many diseases are caused by the presence and actions of specific micro-organisms within the body. The theory was developed and gained gradual acceptance in Europe and the United States from the middle 1800s. It eventually superseded existing miasma and contagion theories of disease and in so doing radically changed the practice of medicine. It remains a guiding theory that underlies contemporary biomedicine.

Awareness of the physical existence of germs preceded the theory by more than two centuries. Discoveries made by several individuals also pointed the way to germ theory. On constructing his first simple microscope in 1677, Antoni van Leeuwenhoek was surprised to see tiny organisms - which he called ‘animalcules’ - in the droplets of water he was examining. He made no connection with disease, and although later scientists observed germs in the blood of people suffering from disease, they suggested that the germs were an effect of the disease, rather than the cause. This fitted with the then popular theory of spontaneous generation.

The observations and actions of Ignaz SemmelweisJoseph Lister and John Snow would retrospectively be acknowledged as contributing to the acceptance of germ theory. But it was the laboratory researches of Louis Pasteur in the 1860s and then Robert Koch in the following decades that provided the scientific proof for germ theory. Their work opened the door to research into the identification of disease-causing germs and potential life-saving treatments.


J Waller, Discovery of the Germ (London: Icon Books, 2004)

M Worboys, Spreading Germs: Disease Theories and Medical Practice in Britain, 1865-1900 (Cambridge: Cambridge University Press, new edition, 2008)



A tiny single-celled living organism too small to be seen by the naked eye. Micro-organisms that cause disease are called bacteria.


A historic expression referring to the transmission of disease between people by means of direct contact.