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Germ theory of disease is the theory that human infectious diseases are caused by specific variety of microorganisms including but not limited to pathogenic bacteria, fungi, protozoa and viruses. The germ theory of disease states that infectious diseases are caused by pathogenic microorganisms inclusive of bacteria, fungi, protozoa and viruses. Prior to the development of the germ theory, people had believed that infectious disease was caused by poisonous air, pollution or bad air. Several epidemic such as cholera (caused by Vibrio cholerae) and Black Death (caused by Yersinia pestis) ravaged the world and caused untold number of deaths in the human race. The germ theory of disease was first proposed by Girolamo Fracastoro in the 15th century. However, the theory was later expanded prior to its final acceptance in the field of medicine by several scientists including Robert Koch.

These infectious agents or microorganisms were known as germs because they infect humans and animals. The germ theory of disease is a fundamental tenet of medicine because it proved that microbes (i.e., pathogenic microorganisms) are the sole causative agents of infectious diseases in humans, animals and plants. However, the germ theory of disease was not accepted for a very long time because it was in opposition to the theory of spontaneous generation – which was widely accepted by many in the 18th century. The scientific proof that microorganisms are the causative agents of infectious diseases was confirmed by the work of Louis Pasteur and Robert Koch.


Other landmarks and discoveries in the field of microbiology are summarized in Table 1.

Table 1. Synopsis of some milestones in microbiology

Discovery of Petri dish (plate)Richard Petri1887
Discovery of nutrient agar (agar-agar)Fanny A. Eilshemius & Dr. Walter Hesse1881
Release of first edition of Bergey’s Manual Professor David Bergey1923
Discovery of Archaea using rRNA analysis Carl Woese & George Fox1977
Discovery of bacterial conjugationEdward Tatum & Joshua Lederberg1946
Discovery of human immunodeficiency virus (HIV)Luc Montagnier1983
Discovery of transposable elementsBarbara McClintock1951
Discovery of immunoglobulin structureRodney Porter1959
First discovery of yellow fever virusWalter Reed1900
Discovery of monoclonal antibodiesGeorges Kohler & Cesar Milstein1975
Discovery of sulfanilamide & sulpha drugsGerhard Domagk1935
Proposed germ theory of diseaseFriedrich Henle1840
Discovery of the ribosomes as sites of protein synthesisSydney Brenne, Francois Jacob and Matthew Meselson1961
Discovery of streptomycinSelman Waksman & Albert Schatz1943
Discovery of bacteriophages Frederick Twort1915
First cultivation of viruses in cell cultureJohn Enders1946
Discovery of diphtheria antitoxinEmil von Behring & Shibasaburo Kitasato1890
Discovery of plasmid DNAStanley Cohen, Annie Chang, Robert Helling and Herbert Boyer1973
First discovery of zoonoses (Babesiosis)Theobald Smith and F.L. Kilbourne1893
Discovery of human blood groupsKarl Landsteiner1901
Crystallization of tobacco mosaic virus (TMV)Wendell Stanley1935
Discovery of first cancer virusFrancis Rous1911
Discovery of lac operonFrancois Jacob, David Perrin, Carmen Sanchez and Jacques Monod1960
Discovery of polymerase chain reaction (PCR)Kary Mullis1985
Discovery of Taq polymeraseThomas Brock & Hudson Freeze1969
Discovery of the genetic codeMarshall Nirenberg & Gobind H. Khorana1966
Discovery of restriction enzymes or endonucleases Hamilton Smith & David Nathans1970
Discovery of DNA sequencingFred Sanger, Steven Niklen & Alan Coulson1977
Discovery of clonal selection theoryMacfarlane F. Burnet1959
Discovery of reverse transcriptaseHoward Temin, David Baltimore & Renato Dulbeccco1969
Discovery of prionsStanley Prusiner1981
Discovery of bacterial transductionJoshua Lederberg & Norton Zinder1952

Louis Pasteur conducted series of experiments that led to the development of pasteurization. His series of experiments also led to the discovery that fermentation is caused by the activities of microorganisms. Pasteur also developed vaccines against a number of diseases caused by microorganisms. Koch developed a series of postulates that help scientists to link a particular microorganism to a particular disease; and he also discovered the causative agents of anthrax (Bacillus anthracis) and tuberculosis (Mycobacterium tuberculosis).      

This very important theory of microbiology (i.e., germ theory of disease) was postulated by Louis Pasteur, a French microbiologist in 1857 when he tried to unmask the reason why beer and wine would go sour, and the cause of disease in silkworms. Silkworms are larvae or caterpillar of silk moth, an economically important insect that produces silk. However, the theory of the causative agents of infectious diseases in man and animals (i.e., the germ theory of disease) has been widely postulated prior to the actual establishment by Louis Pasteur in the 1850’s that microbes were responsible for causing infectious diseases in man.

Some notable microbiologists whose independent research gave impetus to the authenticity of the germ theory of disease included: Girolamo Fracastoro (1483-1553)who postulated that certain diseases are caused by some invisible organisms; Ignaz Semmelweis (1818-1865) who pioneered the use of antimicrobial agents (antiseptics) in labour wards or obstetrical practice to contain microbial contamination and spread in hospital wards; Robert Koch (1843-1910)who developed the famous Koch’s postulates of disease that linked a particular disease to a specific pathogenic microorganism; and Joseph Lister (1827-1912) – who discovered and established the process and importance of antisepsis in medical practice. It is noteworthy to point that prior to Pasteur’s discovery of the germ theory of disease, there were various independent works that tried to establish the link between a disease and a microbe or a pathogenic microorganism.

Of particular interest was Agostino Bassi de Lodi – who showed that some silkworm disease was caused by a fungus; and Berkeley M.J – who showed that the Great Potato Blight disease of Ireland and silkworm disease was caused by a fungus. However, Louis Pasteur was the first to establish the link between a disease and a microbe (even though his findings were based on animal diseases). Pasteur proposed that “a growing living organism was responsible for the spoilage”; thus the name germ actually originated from germination – which indicates that the growth of the microbe in the food (for example, wine and beer) was responsible for the spoilage. This serendipitous discovery by Louis Pasteur – which elaborated the relationship between microorganisms and diseases paved way for the golden era of microbiology which blossomed as a field of the biological sciences.

Pasteur showed in his experiment that microorganisms were responsible for fermentation (i.e., the conversion of sugar to alcohol to produce wine and beer), and that the spoilage of food products was also caused by microbes known as food spoiling microorganisms. Louis Pastures discovery of a heating process known as pasteurization (the application of high heat for a short time) helped to kill microbes in food products including milk, wine and beer. This technique of pasteurization is still applied in the food and other allied industries till date to preserve food and prevent the growth of spoilage microbes in them. Pasteur came up with pasteurization as a method of killing the microbes that caused the spoilage of food, wine and beer.

Louis Pasteur showed in his series of experiments that microbes were responsible for causing the spoilage of wine and beer, and that a microorganism was responsible for causing the silkworm disease but he never established the actual link between microorganisms and human diseases. Prior to the discovery of microorganisms as the main causative agents of infectious diseases, man attributed human illnesses to several non-microbial origins or causes as aforementioned. They even know that there was a link between disease and dirt but could not fathom the actual relationship or cause responsible for it. The germ theory of disease linked microorganisms to particular diseases, and this idea of microbiology has continued to impact medicine and the way in which infectious diseases are treated or managed till date.

Germ theory of disease gives the underlying principles of infection, their causative agents, spread or transmission and the likely ways of containing their untoward effects in a defined human population. Louis Pasteur’s discovery of germs or microorganisms as the causative agents of diseases was further expounded by the works of Robert Koch who developed what is today known as the Koch’s postulate of disease. Robert Koch’s scientific breakthrough came through when he identified the causative agent of anthrax (Bacillus anthracis) in 1875. Koch’s experiment on anthrax, an important disease in cattle as at the time led him to discover the causative agents of tuberculosis and cholera which were both important human diseases. This serendipitous discovery of the causative agents of anthrax, cholera and tuberculosis by Robert Koch gave impetus to the germ theory of disease – which linked microorganisms to particular diseases. Robert Koch was the first to establish the link between a human disease and a microorganism; and his work and that of Pasteur gave the underlying foundation for the germ theory of disease.

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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