What is Germ Theory of Disease?
Today, it is hard for us to fully appreciate the great revolution in medicine known as “germ theory” and the role that animal research played in its development.
It seems impossible that people once believed that foul odors could create disease or that “evil spirits” could cause a person to become ill. We have also forgotten how rare it was for parents to see all of their children survive to adulthood.
The germ theory of disease is the currently accepted scientific theory for many diseases. It states that microorganisms known as pathogens or “germs” can lead to disease. These small organisms, too small to see without magnification, invade humans, other animals, and other living hosts.
Their growth and reproduction within their hosts can cause disease. “Germ” may refer to not just a bacterium but to any type of microorganisms, such as protists or fungi, or even non-living pathogens that can cause diseases, such as viruses, prions, or viroids.
Diseases caused by pathogens are called infectious diseases. Even when a pathogen is the principal cause of a disease, environmental and hereditary factors often influence the severity of the disease, and whether a potential host individual becomes infected when exposed to the pathogen.
Pathogens are diseases that can pass from one individual to another, both in humans and animals. Infectious diseases are caused by biological agents such as pathogenic microorganisms (viruses, bacteria, and fungi) as well as parasites.
Basic forms of germ theory were proposed in the Late Middle Ages by physicians including Ibn Sina in 1025, Girolamo Fracastoro in 1546, and expanded upon by Marcus von Plenciz in 1762. However, such views were held in disdain in Europe, where Galen’s miasma theory remained dominant among scientists and doctors.
Development of Germ Theory of Disease
Support for the idea that microorganisms cause disease that is, the germ theory of disease began to accumulate in the early nineteenth century from diverse fields.
Much of the credit for the development and acceptance of the theory is given to French chemist and microbiologist Louis Pasteur, the English surgeon Joseph Lister, and the German physician Robert Koch.
Agostino Bassi demonstrated in 1835 that a silkworm disease was due to a fungal infection. In 1845 M. J. Berkeley proved that the great potato blight of Ireland was caused by a water mold (then thought to be a fungus), and in 1853 Heinrich de Bary showed that fungi caused cereal crop diseases.
Some of the key Figures in the Development of Germ Theory and their contribution are discussed below.
In addition, to the important individuals described above, Ignaz Semmelweis, John Snow, and Robert Koch are also key figures in the development of the germ theory of disease, and the transmission of microorganisms within a population.
Ignaz Semmelweis
Ignaz Semmelweis was an obstetrician in the mid-1800s who noted a high mortality rate caused by puerperal fever in women giving birth later in the day with the aid of doctors and medical students compared to women who gave birth in the morning and with the aid of midwives.
Through his investigation, he noted that the doctors and medical students helping women to give birth had come from conducting autopsies. Semmelweis asserted that the puerperal fever was caused by a disease spread to the pregnant women via the cadavers in the autopsy rooms.
Following this realization, Semmelweis implemented mandatory handwashing in a chlorinated solution of lime water prior to assisting with births and reduced the childbirth mortality rate from 18% to 2.2%. Despite the success of mandatory handwashing, Semmelweis’s theory was rejected by society during this time.
Gideon Mantell
Gideon Mantell, the Sussex doctor more famous for discovering dinosaur fossils, spent time with his microscope, and speculated in his Thoughts on Animalcules that perhaps “many of the most serious maladies which afflict humanity, are produced by peculiar states of invisible animalcular life”
John Snow
Despite a lack of a fully formulated germ theory of disease, John Snow was one of the first to publish an epidemiological study describing the transmission of cholera via the fecal-oral route.
In addition, in his description of cholera transmission, he rejected miasma theory, describing that cholera was likely transmitted via the accidental ingestion of the fecal matter from the symptomatic individuals.
He further stated that the cholera organism could attach to the intestinal lining, multiply, and induce disease in the next host. With this infectious theory of cholera, Snow advised that water be filtered and boiled prior to consumption based on his epidemiological study of the London cholera outbreak of 1854.
Through his investigation of the cholera epidemic, a public water pump was identified to be the source of the outbreak and once disabled, served to end the epidemic. Snow demonstrated the association between the cases of cholera and the water pump using a dot map.
During his investigation, he found that the pump was suppling water contaminated with sewage, which people were then ingesting. Moreover, it was also found that the location of the pump was situated in close proximity to an inactive cesspit, which was also leaking fecal matter into the pump’s water supply.
Despite the validity of Snow’s findings, he too was met with public resistance regarding the possibility of fecal-oral transmission of disease. Nevertheless, his efforts in describing cholera transmission continue to be regarded as one of the most significant events in the field of public health.
Louis Pasteur
The more formal experiments on the relationship between germ and disease were conducted by Louis Pasteur between the years 1860 and 1864. He discovered the pathology of puerperal fever and the pyogenic vibrio in the blood and suggested using boric acid to kill these microorganisms before and after confinement.
Pasteur further demonstrated between 1860 and 1864 that fermentation and the growth of microorganisms in nutrient broths did not proceed by spontaneous generation.
He exposed freshly boiled broth to air in vessels that contained a filter to stop all particles passing through to the growth medium, and even with no filter at all, with air being admitted via a long tortuous tube that would not pass dust particles.
Nothing grew in the broths: therefore the living organisms that grew in such broths came from outside, as spores on dust, rather than being generated within the broth.
Pasteur discovered that another serious disease of silkworms, pébrine, was caused by a microscopic organism now known as Nosema bombycis (1870).
Pasteur saved France’s silk industry by developing a method to screen silkworms eggs for those that were not infected, a method that is still used today to control this and other silkworm diseases.
Robert Koch
Building on the early work of Louis Pasteur and the germ theory of disease, Robert Koch established the basic scientific requirements used to demonstrate that each specific disease is caused by a specific microorganism. These requirements were based on Koch’s experiments with anthrax isolated from diseased hosts, and are known as “Koch’s Postulates”.
Robert Koch’s postulates are the four criteria designed to assess whether a microorganism causes a disease. As originally stated, the four criteria are:
- The microorganism must be found in diseased but not healthy individuals
- The microorganism must be cultured from the deceased individual
- Inoculation of a healthy individual with the cultured microorganism must recapitulated the disease; and finally
- The microorganism must be re-isolated from the inoculated, diseased individual and matched to the original microorganism.
Koch’s postulates have been critically important in establishing the criteria whereby the scientific community agrees that a microorganism causes a disease.
Top Questions
who proposed the germ theory of disease?
The French chemist and microbiologist Louis Pasteur, the English surgeon Joseph Lister, and the German physician Robert Koch are given much of the credit for the development and acceptance of the theory.
Who opposed germ theory?
Among the major opponents of the germ theory were the men who supported the doctrine of spontaneous generation. The adherents of this ancient and widespread belief thought that germs could arise de novo, without parents, producing disease, which was then propagated from person to person by contagion.
How did Koch prove germ theory?
In the final decades of the 19th century, Koch conclusively established that a particular germ could cause a specific disease. He did this by experimentation with anthrax. Using a microscope, Koch examined the blood of cows that had died of anthrax. He observed rod-shaped bacteria and suspected they caused anthrax.
How did Pasteur’s work influence, Lister and Koch?
Pasteur’s work on the germ theory of disease and aseptic techniques influenced Lister’s study of using disinfectants to prevent the spread of disease in humans. His work also influenced Koch’s experiments that proved microorganisms caused disease and helped influence his development of postulates.
