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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 microorganism or even non-living pathogen that can cause disease, such as protists, fungi, viruses, prions, or viroids.[1] 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.

Basic forms of germ theory were proposed in the late Middle Ages by physicians including Ibn Sina in 1025, Ibn Khatima and Ibn al-Khatib in the 14th century, and 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. The nature of this doctrine prevented them from understanding how diseases actually progressed, with predictable consequences. By the early nineteenth century, smallpox vaccination was commonplace in Europe, though doctors were unaware of how it worked or how to extend the principle to other diseases. Similar treatments had been prevalent in India from just before AD 1000.[2][N 1] A transitional period began in the late 1850s with the work of Louis Pasteur. This work was later extended by Robert Koch in the 1880s. Viruses were discovered in the 1890s. By the end of the 1880s, the miasma theory was struggling to compete with the germ theory of disease. Eventually, a "golden era" of bacteriology ensued, during which the theory quickly led to the identification of the actual organisms that cause many diseases.[3][4]


Miasma theoryEdit

A representation by Robert Seymour of the cholera epidemic depicts the spread of the disease in the form of poisonous air.

The miasma theory was the predominant theory of disease transmission before the germ theory took hold towards the end of the 19th century, and it is no longer accepted as a scientific theory of disease. It held that diseases such as cholera, chlamydia infection, or the Black Death were caused by a miasma (μίασμα, Ancient Greek: "pollution"), a noxious form of "bad air" emanating from rotting organic matter.[5] Miasma was considered to be a poisonous vapor or mist filled with particles from decomposed matter (miasmata) that was identifiable by its foul smell. The theory posited that diseases were the product of environmental factors such as contaminated water, foul air, and poor hygienic conditions. Such infections, according to the theory, were not passed between individuals but would affect those within a locale that gave rise to such vapors.[citation needed]


In Antiquity, the Greek historian Thucydides (c. 460 – c. 400 BC) was the first person to state, in his account of the plague of Athens, that diseases could spread from an infected person to others.[6][7] One theory of the spread of contagious diseases that were not spread by direct contact was that they were spread by "seeds" (Latin: semina) that were present in the air. In his poem, De rerum natura (On the Nature of Things, c. 56 BC), the Roman poet Lucretius (c. 99 BC – c. 55 BC) stated that the world contained various "seeds", some of which could sicken a person if they were inhaled or if they contaminated his food.[8][9] The Roman statesman Marcus Terentius Varro (116–27 BC) wrote, in his Rerum rusticarum libri III (Three Books on Agriculture, 36 BC): "Precautions must also be taken in the neighborhood of swamps […] because there are bred certain minute creatures which cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and there cause serious diseases."[10] The Greek physician Galen (AD 129 – c. 200/c. 216) speculated in his On Initial Causes (c. AD 175) that some patients might have "seeds of fever".[11] In his On the Different Types of Fever (c. AD 175), Galen speculated that plagues were spread by "certain seeds of plague", which were present in the air.[12] And in his Epidemics (c. AD 176–178), Galen explained that patients might relapse during recovery from a fever because some "seed of the disease" lurked in their bodies, which would cause a recurrence of the disease if the patients didn't follow a physician's therapeutic regimen.[13]

During the early Middle Ages, Isidore of Seville (c. 560–636) mentioned "plague-bearing seeds" (pestifera semina) in his On the Nature of Things (c. AD 613).[14] Later in 1345, Tommaso del Garbo (c. 1305–1370) of Bologna, Italy mentioned Galen's "seeds of plague" in his work Commentaria non parum utilia in libros Galeni (Helpful commentaries on the books of Galen).[15]

Late Middle AgesEdit


The Italian scholar and physician Girolamo Fracastoro proposed in 1546 in his book De Contagione et Contagiosis Morbis that epidemic diseases are caused by transferable seed-like entities (seminaria morbi) that transmit infection by direct or indirect contact, or even without contact over long distances. The diseases were categorised based on how they were transmitted, and how long they could lie dormant.


Microorganisms are said to have been first directly observed in the 1670s by Anton van Leeuwenhoek, an early pioneer in microbiology. Yet Athanasius Kircher may have done so prior. When Rome was struck by the bubonic plague in 1656, Kircher spent days on end caring for the sick. Searching for a cure, Kircher observed microorganisms under the microscope and invented the germ theory of disease, which he outlined in his Scrutinium pestis physico-medicum (Rome 1658).[19] Building on Leeuwenhoek's work, physician Nicolas Andry argued in 1700 that microorganisms he called "worms" were responsible for smallpox and other diseases.[20]


In 1762, the Austrian physician Marcus Antonius von Plenciz (1705–1786) published a book titled Opera medico-physica. It outlined a theory of contagion stating that specific animalcules in the soil and the air were responsible for causing specific diseases. Von Plenciz noted the distinction between diseases which are both epidemic and contagious (like measles and dysentry), and diseases which are contagious but not epidemic (like rabies and leprosy).[22] The book cites Anton van Leeuwenhoek to show how ubiquitous such animalcules are, and was unique for describing the presence of germs in ulcerating wounds. Ultimately, the theory espoused by von Plenciz was not accepted by the scientific community.

Agostino BassiEdit

The Italian Agostino Bassi was the first person to prove that a disease was caused by a microorganism when he conducted a series of experiments between 1808 and 1813, demonstrating that a "vegetable parasite" caused a disease in silkworms known as calcinaccio—this disease was devastating the French silk industry at the time. The "vegetable parasite" is now known to be a fungus pathogenic to insects called Beauveria bassiana (named after Bassi).

Ignaz SemmelweisEdit

Ignaz Semmelweis, a Hungarian obstetrician working at the Vienna General Hospital (Allgemeines Krankenhaus) in 1847, noticed the dramatically high maternal mortality from puerperal fever following births assisted by doctors and medical students. However, those attended by midwives were relatively safe. Investigating further, Semmelweis made the connection between puerperal fever and examinations of delivering women by doctors, and further realized that these physicians had usually come directly from autopsies. Asserting that puerperal fever was a contagious disease and that matter from autopsies were implicated in its development, Semmelweis made doctors wash their hands with chlorinated lime water before examining pregnant women. He then documented a sudden reduction in the mortality rate from 18% to 2.2% over a period of a year. Despite this evidence, he and his theories were rejected by most of the contemporary medical establishment.

Gideon MantellEdit

Gideon Mantell, the Sussex doctor more famous for discovering dinosaur fossils, spent time with his microscope, and speculated in his Thoughts On Animalcules (1850) that perhaps "many of the most serious maladies which afflict humanity, are produced by peculiar states of invisible animalcular life".[23]

John SnowEdit




— John Snow (1849)


— John Snow (1855)






After the cholera epidemic had subsided, government officials replaced the handle on the Broad Street pump. They had responded only to the urgent threat posed to the population, and afterward they rejected Snow's theory. To accept his proposal would have meant accepting the fecal-oral method transmission of disease, which they dismissed.[25]

Louis PasteurEdit


The more formal experiments on the relationship between germ and disease were conducted by Louis Pasteur between the year 1860 and 1864. He discovered the pathology of the puerperal fever[26] and the pyogenic vibrio in the blood, and suggested using boric acid to kill these microorganisms before and after confinement.


Pasteur discovered that another serious disease of silkworms, pébrine, was caused by a small 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 KochEdit


Koch's postulates were developed in the 19th century as general guidelines to identify pathogens that could be isolated with the techniques of the day.[27] Even in Koch's time, it was recognized that some infectious agents were clearly responsible for disease even though they did not fulfill all of the postulates.[28][29] Attempts to rigidly apply Koch's postulates to the diagnosis of viral diseases in the late 19th century, at a time when viruses could not be seen or isolated in culture, may have impeded the early development of the field of virology.[30][31] Currently, a number of infectious agents are accepted as the cause of disease despite their not fulfilling all of Koch's postulates.[32] Therefore, while Koch's postulates retain historical importance and continue to inform the approach to microbiologic diagnosis, fulfillment of all four postulates is not required to demonstrate causality.


Koch's postulates:




The second postulate may also be suspended for certain microorganisms or entities that cannot (at the present time) be grown in pure culture, such as prions responsible for Creutzfeldt–Jakob disease.[34] In summary, a body of evidence that satisfies Koch's postulates is sufficient but not necessary to establish causation.



See alsoEdit




  1. ^ "germ – definition of germ in English from the Oxford dictionary". Archived from the original on 6 April 2016. Retrieved 5 April 2016.
  2. ^ Henderson, Donald A; Moss, Bernard (1999). Smallpox and Vaccinia, Vaccines. W.B. Saunders Company. Archived from the original on 21 May 2017. Retrieved 31 August 2017.
  3. ^ "Brief History During the Snow Era". Archived from the original on 17 January 2017. Retrieved 1 January 2016.
  4. ^ "Germ Theory". Archived from the original on 31 January 2016. Retrieved 1 January 2016.
  11. ^ Nutton (1983), p. 4
  12. ^ Nutton (1983), p. 6
  13. ^ Nutton (1983), p. 7
  14. ^ Nutton (1983), p. 20
  15. ^ Nutton (1983), p. 21
  19. ^ "The Life and Work of Athanaseus Kircher, S.J." Archived from the original on 17 April 2016. Retrieved 18 April 2016.
  21. ^ Melvin Santer, "Richard Bradley: A Unified, Living Agent Theory of the Cause of Infectious Diseases of Plants, Animals, and Humans in the First Decades of the 18th Century", in Perspectives in Biology and Medicine, Volume 52, Number 4, Autumn, 2009, pp. 566–78
  23. ^ From p. 90 of "The invisible world revealed by the microscope or, thoughts on animalcules.", second edition, 1850 (May have appeared in first edition, too. (Revise date in article to 1846, if so.))
  26. ^ Pasteur, Louis (1880) [May 1880]. "(translated from French)" [On the extension of the germ theory to the etiology of certain common diseases]. Comptes Rendus de l'Académie des Sciences. XC. Ernst, H.C. (trans). pp. 1033–44.
  29. ^ a b Koch Robert (1893). "Über den augenblicklichen Stand der bakteriologischen Choleradiagnose". Zeitschrift für Hygiene und Infektionskrankheiten (in German). 14: 319–33. doi:10.1007/BF02284324.
  33. ^ Falkow S (1988). "Molecular Koch's postulates applied to microbial pathogenicity". Reviews of Infectious Diseases. 10 (Suppl 2): S274–76. doi:10.1093/cid/10.Supplement_2.S274. PMID 3055197.
  34. ^ Inglis TJ (November 2007). "Principia aetiologica: taking causality beyond Koch's postulates". Journal of Medical Microbiology. 56 (Pt 11): 1419–22. doi:10.1099/jmm.0.47179-0. PMID 17965339. Archived from the original on 5 March 2010. Retrieved 3 December 2012.

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