Germ theory of disease

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 highly controversial when first proposed, it is now a cornerstone of modern medicine and clinical microbiology, leading to such important innovations as antibiotics and hygienic practices.

History
The ancient historical view was that disease was spontaneously generated instead of being created by microorganisms which grow by reproduction. The Atharvaveda is the first ancient text dealing with medicine. It identifies the causes of disease as living causative agents such as the yatudhānya, the kimīdi, the kṛimi and the durṇama. The atharvāns seek to kill them with a variety of drugs in order to counter the disease (see XIX.34.9). One of the earliest western 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: ...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.

In The Canon of Medicine (1020), Abū Alī ibn Sīnā (Avicenna) stated that bodily secretion is contaminated by foul foreign earthly bodies before being infected. He also discovered the contagious nature of tuberculosis and other infectious diseases, and introduced quarantine as a means of limiting the spread of contagious diseases.

When the Black Death bubonic plague reached al-Andalus in the 14th century, Ibn Khatima hypothesized that infectious diseases are caused by "minute bodies" which enter the human body and cause disease. Another 14th century Andalusian physician, Ibn al-Khatib, wrote a treatise called On the Plague, in which he stated:

""The existence of contagion is established by experience, investigation, the evidence of the senses and trustworthy reports. These facts constitute a sound argument. The fact of infection becomes clear to the investigator who notices how he who establishes contact with the aftlicted gets the disease, whereas he who is not in contact remains safe, and how transmission is affected through garments, vessels and earrings.""

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.

Italian physician Francesco Redi provided proof against spontaneous generation. He devised an experiment in 1668 where he used three jars. He placed a meat loaf in each of the three jars. He had one of the jars 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 jar was covered by maggots, and the jar covered with gauze had maggots on the surface of the gauze. However, the tightly sealed jar 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 spontaneous generation is not a plausible theory.

Microorganisms were first directly 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 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.

John Snow contributed to the formation of the germ theory when he traced the source of the 1854 cholera outbreak in the Soho neighborhood of London. The statistical analysis of the affected cases showed that the outbreak was not consistent with the miasma theory which was prevalent at the time. 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, not coincidentally, was at the center of the outbreak.

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

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 Koch's third postulate, 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, socioeconomic status, nutritional and/or immunologic status, or lifestyle are more important than germs themselves. Many people with same or similar exposure to same microbes, may not all get similar infection and outcome under normal environment. It can be immune defense strength dependent. But immune strength, susceptibilities and sensitivities related to particular microbes can also be dependent on instabilities and imbalances in the biochemistry of an individual. As such, it can be thought that any deviation from homeostasis can also be a reason to invite and get any infection somewhat justify "miasma" or substance based theory. However, "miasma" has never been observed scientifically, and this hypothesis is not supported by existing evidence.