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Snow's 1849 recommendation that water be "filtered and boiled before it is used" is one of the first practical app

Snow's 1849 recommendation that water be "filtered and boiled before it is used" is one of the first practical applications of germ theory in the area of public health and is the antecedent to the modern boil-water advisory.

In 1855 he published a second edition of his article, documenting his more elaborate in

In 1855 he published a second edition of his article, documenting his more elaborate investigation of the effect of the water supply in the Soho, London epidemic of 1854.

By talking to local residents, he identified the source of the outbreak as the public water pump on Broad Street (now Broadwick Street). 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. This action has been commonly credited as ending the outbreak, but Snow observed that the epidemic may have already been in rapid decline.[28]

Snow later used a dot map to illustrate the cluster of cholera cases around the pump. He also used statistics to illustrate the connection between the quality of the water source and cholera cases, showing that the Southwark and Vauxhall Waterworks Company was taking water from sewage-polluted sections of the Thames and delivering the water to homes, leading to an increased incidence of cholera. He also showed that houses supplied by cleaner upriver water from a Lambeth Waterworks Company supply at Seething Wells had far lower cholera mortality rates.[29] Snow's study was a major event in the history of public health and geography. It is regarded as one of the founding events of the science of epidemiology.

Later, researchers discovered that this public well had been dug only three feet from an old cesspit, which had begun to leak fecal bacteria.[citation needed] The diaper of a baby, who had contracted cholera from another source, had been washed into this cesspit. Its opening was originally under a nearby house, which had been rebuilt farther away after a fire. The city had widened the street and the cesspit was lost. It was common at the time to have a cesspit under most homes. Most families tried to have their raw sewage collected and dumped in the Thames to prevent their cesspit from filling faster than the sewage could decompose into the soil.

After the cholera epidemic had subsided, government officials replaced the handle on the Broad Street pump. They had responded only to the urgent threat posed to the population, and afterward, they rejected Snow's theory. To accept his proposal would have meant accepting the fecal-oral method transmission of disease, which they dismissed.[30]

The more formal experiments on the relationship between germ and disease were conducted by Louis Pasteur between the years 1860 and 1864. He discovered the pathology of the puerperal fever[31] and the pyogenic vibrio in the blood, and suggested using boric acid to kill these microorganisms before and after confinement.

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.

Pasteur discovered that another serious disease of silkworms, pébrine, was caused by a microscopic organism now known as Nosema bombycis (1870). Pasteur saved France's silk industry by developing a method to screen silkworms eggs for those that were not infected, a method that is still used today to control this and other silkworm diseases.

Robert Koch[<

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.

Pasteur discovered that another serious disease of silkworms, pébrine, was caused by a microscopic organism now known as Nosema bombycis (1870). Pasteur saved France's silk industry by developing a method to screen silkworms eggs for those that were not infected, a method that is still used today to control this and other silkworm diseases.

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. These postulates grew out of his seminal work with anthrax using purified cultures of the pathogen that had been isolated from diseased animals.

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.[32] Even in Koch's time, it was recognized that some infectious agents were clearly re

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.[32] 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.[33][34] 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.[35][36] Currently, a number of infectious agents are accepted as the cause of disease despite their not fulfilling all of Koch's postulates.[37] 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.

Koch's postulates have also influenced scientists who examine microbial pathogenesis from a molecular point of view. In the 1980s, a molecular version of Koch's postulates was developed to guide the identification of microbial genes encoding virulence factors.[38]

Koch's postulates:

However, Koch abandoned the universalist requirement of the first postulate altogether when he discovered asymptomatic carriers of cholera[34] and, later, of typhoid fever. Asymptomatic or subclinical infection carriers are now known to be a common feature of many infectious diseases, especially viruses such as polio, herpes simplex, HIV, and hepatitis C. As a specific example, all doctors and virologists agree that poliovirus causes paralysis in just a few infected subjects, and the success of the polio vaccine in preventing disease supports the conviction that the poliovirus is the causative agent.

The third postulate specifies "should", not "must", because as Koch himself proved in regard to both tuberculosis and cholera,[33] not all organisms exposed to an infectious agent will acquire the infection. Noninfection may be due to such factors as general health and proper immune functioning; acquired immunity from previous exposure or vaccination; or genetic immunity, as with the resistance to malaria confe

The third postulate specifies "should", not "must", because as Koch himself proved in regard to both tuberculosis and cholera,[33] not all organisms exposed to an infectious agent will acquire the infection. Noninfection may be due to such factors as general health and proper immune functioning; acquired immunity from previous exposure or vaccination; or genetic immunity, as with the resistance to malaria conferred by possessing at least one sickle cell allele.

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.[39] 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.

See also