Germ theory of disease
The germ theory of disease is the currently accepted scientific theory for many diseases. It states that microorganisms known as pathogens or "germs" can cause disease. These small organisms, which are too small to be seen without magnification, invade animals, plants, and even bacteria. Their growth and reproduction within their hosts can cause disease. "Germ" refers not just to bacteria but to any type of microorganism, such as protists or fungi, or other pathogens, including parasites, 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 disease-causing agents that can pass from one individual to another, across multiple domains of life.
Basic forms of germ theory were proposed by 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.
By the early 19th century, the first vaccine, smallpox vaccination, was commonplace in Europe, though doctors were unaware of how it worked or how to extend the principle to other diseases. 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. By the end of that decade, the miasma theory was struggling to compete with the germ theory of disease. Viruses were initially discovered in the 1890s. Eventually, a "golden era" of bacteriology ensued, during which the germ theory quickly led to the identification of the actual organisms that cause many diseases.
Miasma theory
The miasma theory was the predominant theory of disease transmission before the germ theory took hold towards the end of the 19th century; it is no longer accepted as a correct explanation for disease by the scientific community. It held that diseases such as cholera, chlamydia infection, or the Black Death were caused by a , 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 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.Development of germ theory
Greece and Rome
In Antiquity, the Greek historian Thucydides was the first person to write, 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 spore-like "seeds" that were present in and dispersible through the air. In his poem, De rerum natura, the Roman poet Lucretius stated that the world contained various "seeds", some of which could sicken a person if they were inhaled or ingested.
The Roman statesman Marcus Terentius Varro wrote, in his Rerum rusticarum libri III : "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 speculated in his On Initial Causes that some patients might have "seeds of fever". In his On the Different Types of Fever, Galen speculated that plagues were spread by "certain seeds of plague", which were present in the air. And in his Epidemics, Galen explained that patients might relapse during recovery from fever because some "seed of the disease" lurked in their bodies, which would cause a recurrence of the disease if the patients did not follow a physician's therapeutic regimen.
The Middle Ages
A hybrid form of miasma and contagion theory was proposed by Persian physician Ibn Sina in The Canon of Medicine. He mentioned that people can transmit disease to others by breath, noted contagion with tuberculosis, and discussed the transmission of disease through water and dirt.During the early Middle Ages, Isidore of Seville mentioned "plague-bearing seeds" in his On the Nature of Things. Later, in 1345, Tommaso del Garbo of Bologna, Italy mentioned Galen's "seeds of plague" in his work Commentaria non-parum utilia in libros Galeni.
The 16th century Reformer Martin Luther appears to have had some idea of the contagion theory, commenting, "I have survived three plagues and visited several people who had two plague spots which I touched. But it did not hurt me, thank God. Afterwards when I returned home, I took up Margaret,", "who was then a baby, and put my unwashed hands on her face, because I had forgotten; otherwise I should not have done it, which would have been tempting God." In 1546, Italian physician Girolamo Fracastoro published De Contagione et Contagiosis Morbis, a set of three books covering the nature of contagious diseases, categorization of major pathogens, and theories on preventing and treating these conditions. Fracastoro blamed "seeds of disease" that propagate through direct contact with an infected host, indirect contact with fomites, or through particles in the air.
The Early Modern Period
In 1668, Italian physician Francesco Redi published experimental evidence rejecting spontaneous generation, the theory that living creatures arise from nonliving matter. He observed that maggots only arose from rotting meat that was uncovered. When meat was left in jars covered by gauze, the maggots would instead appear on the gauze's surface, later understood as rotting meat's smell passing through the mesh to attract flies that laid eggs.Microorganisms are said to have been first directly observed in the 1670s by Anton van Leeuwenhoek, an early pioneer in microbiology, considered "the Father of Microbiology". Leeuwenhoek is said to be the first to see and describe bacteria in 1674, yeast cells, the teeming life in a drop of water, and the circulation of blood corpuscles in capillaries. The word "bacteria" did not exist yet, so he called these microscopic living organisms "animalcules", meaning "little animals". Those "very little animalcules" he was able to isolate from different sources, such as rainwater, pond and well water, and the human mouth and intestine.
Yet German Jesuit priest and scholar Athanasius Kircher may have observed such microorganisms prior to this. One of his books written in 1646 contains a chapter in Latin, which reads in translation: "Concerning the wonderful structure of things in nature, investigated by microscope...who would believe that vinegar and milk abound with an innumerable multitude of worms." Kircher defined the invisible organisms found in decaying bodies, meat, milk, and secretions as "worms." His studies with the microscope led him to the belief, which he was possibly the first to hold, that disease and putrefaction, or decay were caused by the presence of invisible living bodies, writing that "a number of things might be discovered in the blood of fever patients." When Rome was struck by the bubonic plague in 1656, Kircher investigated the blood of plague victims under the microscope. He noted the presence of "little worms" or "animalcules" in the blood and concluded that the disease was caused by microorganisms.
Kircher was the first to attribute infectious disease to a microscopic pathogen, inventing the germ theory of disease, which he outlined in his Scrutinium Physico-Medicum, published in Rome in 1658. Kircher's conclusion that disease was caused by microorganisms was correct, although it is likely that what he saw under the microscope were in fact red or white blood cells and not the plague agent itself. Kircher also proposed hygienic measures to prevent the spread of disease, such as isolation, quarantine, burning clothes worn by the infected, and wearing facemasks to prevent the inhalation of germs. It was Kircher who first proposed that living beings enter and exist in the blood.
In the 18th century, more proposals were made, but struggled to catch on. In 1700, physician Nicolas Andry argued that microorganisms he called "worms" were responsible for smallpox and other diseases. In 1720, Richard Bradley theorised that the plague and "all pestilential distempers" were caused by "poisonous insects", living creatures viewable only with the help of microscopes.
In 1762, the Austrian physician Marcus Antonius von Plenciz 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, and diseases which are contagious but not epidemic. 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.