germ theory

For several centuries, people thought diseases were caused by wandering clouds of poisonous vapor.

We now know that this theory is pretty ridiculous, and that diseases are caused by specific bacteria. But how did we get to this new idea of germ theory? Tien Nguyen describes the work of several scientists who discredited a widely accepted theory in a way that was beneficial to human health.

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 It eventually superseded existing miasma and contagion theories of disease and in so doing radically changed the practice of medicine.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 bio-medicine.

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 Semmelweis, Joseph 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.

Four types of germs are:  bacteria, viruses, fungi, and protozoa.