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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 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.

Histories

The ancient historical view was that disease was spontaneously generated instead of being created by microorganisms which grow by reproduction. The Atharvaveda, a sacred text of Hinduism, is the one of the earliest ancient texts 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:

In The Canon of Medicine (1025), 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-Andalusmarker 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:

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

Microorganisms were first directly observed by Anton van Leeuwenhoek, who is considered the father of microbiology. Ignaz Semmelweis was a Hungarianmarker obstetrician working at Vienna's Allgemeines Krankenhausmarker in 1847, when he noticed the dramatically high incidence of death from puerperal fever among women who delivered at the hospital with the help of the doctors and medical students. Births attended by the 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 water and lime before examining pregnant women, thereby reducing mortality from childbirth to less than 2% at his hospital. Nevertheless, he and his theories were viciously attacked by most of the Viennese medical establishment.

John Snow contributed to the formation of the germ theory when he traced the source of the 1854 cholera outbreak in Soho, 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 miasma 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 was at the geographical center of the outbreak.

Italian physician Francesco Redi provided early evidence 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. 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 published in 1890, and derived from his work demonstrating that 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.

In the 1870's Joseph Lister was instrumental in developing practical applications of the germ theory of disease with respect to surgical techniques.

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