Cholera is a contagious bacterial disease that infects the intestines and causes severe diarrhea. The bacteria secretes a toxin which has the effect of opening up ion exchange in the intestinal cells that usually retain water in the bloodstream. Instead, the channel remains open, allowing fluids in the body to flood the intestine, even as the bloodstream becomes dehydrated. Cholera will run its course without treatment, but patients usually die of dehydration as the result of the body's fluids being emptied into the intestines.

The bacteria is released in the diarrhea and infection can occur when water containing the bacteria is consumed by another person, or the bacteria infests fish that pass through sewage. As a result, cholera is rare in places with effective sewage treatment. However, it spreads quickly throughout a population where drinking water is contaminated by sewage.

Cholera was first identified in India in the early 19th century and spread to Europe and elsewhere. The usual pattern of the disease was to infect thousands of people very quickly before dying out within a population. However, where sanitary conditions were poor, a single infected individual could quickly pass on the illness to hundreds of others.

The investigation of cholera epidemics in the mid 19th century gave rise to the modern science of epidemiology. British scientists of the day noted that the disease was more prevalent at locations which were below 32 feet elevation (precisely the distance water can be raised with a pump) above the River Thames – at that time always polluted with raw sewage. They also tracked where cases occurred, often finding that sufferers used a single contaminated pump. As a result, London was provided with water drawn from sources well away from the river, sewers were placed throughout the city to draw all human waste into the river, and the waste was stored until the tide was going out, taking it away from the city.

The cause of the disease was identified as a bacteria by Robert Koch. It led to one of the greatest scientific debates of the day and several scientists refused to believe a single organism was the source of the disease until one skeptic consumed a sample Koch had prepared.

Although cholera can be treated with antibiotics, the primary concern is to replace fluids in the body. In most cases, this alone will keep the patient alive until the disease runs its course. Giving the patient clean water as long as they can stomach it is preferred and, in extreme cases, intravenous solution must be given.

It was noted that the parents of sufferers of cystic fibrosis are much less likely to contract cholera. It is believed they have a gene which prevents the cholera bacteria from opening up the ion gates in the intestine, making them naturally resistant to the disease. However, having two copies of the gene (homozygous) leads to developing cystic fibrosis.

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