Germ Theory
The Germ Theory, which emerged in the late 19th century, demonstrated that microscopic germs caused most human infectious diseases. The germs involved included bacteria, viruses, fungi, protozoa, and prions. Louis Pasteur (1822-1895), a French chemist and microbiologist, and Robert Koch (1843-1910), a German physician and microbiologist, are credited with the discovery of the germ theory in the 1860s-1880s.
Regarded as the most important discovery in the history of medicine, the germ theory challenged the medical profession to reevaluate how disease was thought about, offered possibilities for both the prevention and treatment of disease, as well as the discovery and implementation of new technologies to combat disease.
Previously, doctors assumed that disease was an internal process of the human body especially Hippocrates' long-standing four humors theory notion that excesses or deficiencies of four bodily fluids (blood, phlegm, yellow and black biles) led to illness and disease. The germ theory contradicted that idea by separating the disease from the afflicted persons. Furthermore, the new theory ushered in a regimented way of classifying diseases (nosology) according to the type of microorganisms causing the disease.
Historical Theories of Disease
Prior to the discovery of the germ theory, various theories were advanced as possible explanations for illness and disease in humans. The earliest theory was the miasma theory attributed to Hippocrates (460-370 BCE), a Greek physician. Derived from the Greek word meaning pollution or "bad air", the miasma theory suggested that decomposing particles from organic materials, plants or animals, poisoned the air. Although easily detected by smell, people who inhaled the "bad air" would become ill. Additionally, planetary movements, disturbances to the Earth, poor hygiene, and polluted water often contributed to miasma. Attempts to remove waste along with cleanliness were thought necessary to improve the atmosphere to avoid infection and disease.
Aristotle (384-322 BCE), a Greek philosopher, offered the spontaneous generation of disease. It was possible, Aristotle thought, for living organisms to spring from non-living matter. Furthermore, this process, like maggots appearing from dead flesh, was a regular and natural phenomenon.
Galen (129-216 CE), a Roman physician, extended Hippocrates' earlier speculation about the imbalance of bodily fluids as the cause of disease. Galen attached each of the four humors to a particular season characterized by hot, cold, dry, and wet. For example, colds and flues occurred most often during cold and wet weather. Any change in the weather or season could upset the balance of the four humors so treatments were devised to restore said balance e.g., purges, bloodletting, enemas, and vomits. These ancient theories dominated Western medical thinking about illness until the 19th century.
Another theory of the origin of diseases referred to supernatural causes. A person's sins resulted in contracting a disease or illness as a punishment from the gods or God. Ghosts, demons, and evil spirits also possessed the ability to afflict a person with illness. Magic, divination, spells, exorcism, and various drugs were used to diagnose and treat illness. It fell to a variety of healers – shamans, priests, diviners, medicine men – to drive away the evil spirits. Illness as a punishment for sins, as well as a test of faith, was later offered by Christian theologians as an explanation for disease.
Additional theories on the origin of diseases continued to emerge. Girolamo Fracastoro (1476-1553), an Italian physician, is credited with first using the word "contagion" when describing the transmission of illness. His "seeds of disease" theory argued that disease could be spread by direct or indirect contact or over long distances through no contact at all. A German chemist, Justus von Liebig (1803-1873), one of the early founders of organic chemistry, suggested that as a result of a chemical process from decaying organic matter, disease simply emerged in the blood (the body's "chemical factory").
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