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The '''germ theory of disease''', also called the '''pathogenic theory of medicine''', is a [[theory]] that proposes that microorganisms are the cause of many diseases. Although controversial when first proposed, it is now a cornerstone of modern medicine and clinical [[microbiology]], leading to such important innovations as [[antibiotics]] and [[Hygiene|hygienic practices]].

== Historical view of disease ==
The ancient historical view was that disease was [[spontaneous generation|spontaneously generated]] instead of being created by microorganisms which grow by [[reproduction]].  One of the earliest references to this latter theory appears in ''On Agriculture'' by [[Marcus Terentius Varro]] (published in [[36 BC]]) wherein there is a warning about locating a homestead in the proximity of swamps which reads "...and 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." (''Varro On Agriculture'' 1,xii Loeb) 

[[Girolamo Fracastoro]] proposed in 1546 that epidemic diseases are caused by transferable seedlike entities that could transmit infection by direct or indirect contact or even without contact over long distances.

Microorganisms were first observed by [[Anton van Leeuwenhoek]], who is considered the father of [[microbiology]].

The Italian [[Agostino Bassi]] is often credited with having stated the germ theory of disease for the first time, based on his observations on the lethal and epidemic [[Beauveria bassiana|muscardine]] disease of [[silkworms]]. In 1835 he specifically blamed the deaths of the insects on a contagious, living agent, that was visible to the naked eye as powdery spore masses; this microscopic fungus was subsequently called ''[[Beauveria bassiana]]'' in his honor. 

Italian physician [[Francesco Redi]] provided proof against spontaneous generation. He devised an experiment where he used three flasks. He placed a meat loaf in each of the three flasks. He had one of the flasks open, another one tightly sealed, and the last one covered with gauze. After a few days, he observed that the meat loaf in the open flask was covered by maggots, and the flask covered with gauze had maggots on the surface of the gauze. However, the tightly sealed flask had no maggots inside or outside it. He also noticed that the maggots were only found on surfaces that were accessible by flies. From this he concluded that [[abiogenesis]] is not a plausible theory.

[[John Snow (physician)|John Snow]] contributed to the formation of the germ theory when he traced the source of the 1854 [[cholera]] outbreak in the Soho neighbourhood of London. The statistical analysis of the affected cases showed that the outbreak was not consistent with the prevailing at the time [[Miasma theory of disease|miasma theory]]. Contrary to the contagion model, he identified drinking water as the vessel for transmission of the disease. He found that cases occurred in the homes which obtained their water from the [[Broad Street pump]], which coincidentally was at the epicentre of the outbreak. 

[[Louis Pasteur]] further demonstrated between 1860 and 1864 that [[fermentation (food)|fermentation]] and the growth of microorganisms in nutrient broths did not proceed by spontaneous generation. He exposed freshly boiled broths 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. 

[[Robert Koch]] was the first scientist to devise a series of proofs used to verify the Germ Theory of Disease.  [[Koch's postulates]] were first used in 1875 to demonstrate anthrax was caused by the bacterium ''[[Bacillus anthracis]]''.  These postulates are still used today to help determine if a newly discovered disease is caused by a microorganism.

==Reception of the germ theory of disease ==
"Science proceeds by the death of scientists", and although Pasteur's demonstration essentially solved the question, the germ theory was not universally immediately accepted.  The germ theory of disease was a theoretical foundation of [[epidemiology|infectious disease epidemiology]], the development and use of anti-microbial and [[antibiotic]] drugs, further [[vaccines]] after the empirically derived one for smallpox, [[infection control]] in hospitals, and [[public]] [[sanitation]].  The germ theory provided a rationale for some existing practices and enabled others to be established.  

==Criticism==
Though no one seriously disputes the germ theory outright, there are some who believe that it is incomplete as a theory of disease.  The most commonly cited reason is the clinical inaccuracy of the 3rd of [[Koch's postulates]], which states that any susceptible animal infected with a pathogenic microbe should express symptoms.  Koch himself later recanted this postulate after evidence showed asymptomatic carriers of [[typhoid]] and [[cholera]].  

Others theories of disease accentuate the host resistance factors, arguing that germs are too ubiquitous to be viewed as the 'cause' of disease, even if they are a necessary component of disease.  These approaches typically accept the mechanics of the germ theory, but emphasize that [[heredity]], [[public health]], [[Socioeconomics|socioeconomic]] [[status]], [[nutrition|nutritional]] and/or [[Immune system|immunologic]] status, or [[lifestyle]] are more important than germs themselves.

Outside of science, previous theories ascribing disease to supernatural or divine agents also continue to have proponents.

==References==
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