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Friend or foe? We need water and air to live, but both can carry - and disperse - dirt

Cholera illustration from Punch magazine

Before Louis Pasteur and Robert Koch formulated 'germ theory' in the 19th century, people thought that diseases were transmitted by foul-smelling air, known as a miasma. When cholera first broke out in London in 1832, the belief that it was transmitted through the air - not the water system - had devastating consequences. To get rid of cholera from the air, sewage was diverted into the river Thames, London's main source of drinking water. After John Snow proved the disease was waterborne, a new sewer network was built in London, and today throughout the UK tap water is safe to drink.

Watch a film illustrating the movement and agglutination of both typhoid and cholera bacilli (c.1910).

However, in the over-populated slums of low-income countries, poor sanitation and inadequate sewerage systems mean cholera and other water-borne diseases are still thriving. Wellcome Trust-funded researchers in Brazil have shown that Salvador's city-wide project to improve the sewer network and water supply reduced cases of diarrhoea in children by over 40 per cent in some areas - evidence that city-wide sanitation is an extremely effective intervention against water-borne disease.

At the start of 2008, the International Year of Sanitation, nearly two-thirds of people living sub-Saharan Africa didn't have a toilet in their homes. The Scientists Networked for Outcomes from Water and Sanitation (SNOWS) consortium of universities across the continent and beyond, funded by the Trust's African Institutions Initiative, aims to begin to redress the balance by building African capacity for interdisciplinary research on water supply, sanitation and environmental health.

Water carries disease and death, but it is also essential to life: without it all our organs and bodily functions would fail. In England from the 1840s onwards, belief in the curative powers of water drove the rapid rise of new hydropathic resorts alongside the traditional spas. Trust-funded historian Hilary Marland notes that these coincided with the growth of industry and were presented as healing environments in contrast to polluted, unhealthy urban communities. Home treatments gave the patient a central role in the therapeutic process and ushered in a new era of self-help. After three months of treatment at Malvern, Charles Darwin built an outdoor douche and bath in his garden, which he used daily for five years. In the latter of the 20th century the NHS withdrew its support for these therapies, which are now largely delivered by the private leisure sector.

As with pure water, the curative powers of clean, fresh air were highly valued in 19th-century Europe. Mountain sanatoriums opened in the hope that pure, invigorating mountain air and low atmospheric pressure would increase blood levels of oxygen and lung capacity in people with tuberculosis, as well as containing outbreaks by isolating carriers.

Mechanisation took over from fresh air, and people with TB are nowadays isolated in mechanically ventilated negative-pressure rooms. However, fresh air may be making a comeback. In 2007, in a study of eight hospitals in Lima, Peru, Trust-funded researchers at Imperial College London showed that opening windows was a better form of ventilation and is likely to reduce transmission of TB.

Infections aren't the only threat from 'dirty' air. In the poorest parts of the world, traditional solid-fuel household stoves are used for both cooking and heating, and the levels of indoor pollutants they generate increase the risk of respiratory and heart diseases in adults.

Last year an international consortium of researchers exploring the health effects of policies to reduce greenhouse gas emissions found that, in India, a ten-year programme to introduce 150 million low-emission cooking stoves would dramatically lower rates of these diseases.

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