The germ theory of disease is the currently accepted scientific theory of disease. It states that many diseases are caused by microorganisms. “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. Microorganisms that cause disease are called pathogens, and the diseases they cause are called infectious diseases. 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. Viruses were discovered in the 1890s. Miasma theory 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. 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. 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. 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. 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. 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.” The Greek physician Galen (AD 129 – ca. 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. 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. 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). Building on Leeuwenhoek’s work, physician Nicolas Andry argued in 1700 that microorganisms he called “worms” were responsible for smallpox and other diseases. It outlined a theory of contagion stating that specific animalcules in the soil and the air were responsible for causing specific diseases. 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. 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. Even though the germ theory of disease pioneered by Girolamo Fracastoro had not yet achieved full development or widespread currency, Snow demonstrated a clear understanding of germ theory in his writings. He first published his theory in an 1849 essay On the Mode of Communication of Cholera, in which he correctly suggested that the fecal-oral route was the mode of communication, and that the disease replicated itself in the lower intestines. Although Snow’s chemical and microscope examination of a water sample from the Broad Street pump did not conclusively prove its danger, his studies of the pattern of the disease were convincing enough to persuade the local council to disable the well pump by removing its handle. These experiments were important pieces of evidence supporting the idea of germ theory of disease. The more formal experiments on the relationship between germ and disease were conducted by Louis Pasteur between the year 1860 and 1864. Pasteur discovered that another serious disease of silkworms, pébrine, was caused by a small microscopic organism now known as Nosema bombycis (1870). Robert Koch is known for developing four basic criteria (known as Koch’s postulates) for demonstrating, in a scientifically sound manner, that a disease is caused by a particular organism. 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. 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. 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. Currently, a number of infectious agents are accepted as the cause of disease despite their not fulfilling all of Koch’s postulates. 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. The cultured microorganism should cause disease when introduced into a healthy organism. 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. In summary, a body of evidence that satisfies Koch’s postulates is sufficient but not necessary to establish causation. In the 1870s, Joseph Lister was instrumental in developing practical applications of the germ theory of disease with respect to sanitation in medical settings and aseptic surgical techniques—partly through the use of carbolic acid (phenol) as an antiseptic.
Source: Germ theory of disease