germ theory was not established at this point (Louis Pasteur did not propose it until 1861). Snow did not understand the mechanism by which disease was transmitted, but the evidence led him to believe that it was not due to breathing foul air. Based on the pattern of illness among residents, Snow hypothesized that cholera was spread by an agent in contaminated water. He first published his theory in 1849, in an essay titled "On the Mode of Communication of Cholera". In 1855 he published a second edition, including a more elaborate investigation of the effect of the water supply in the 1854 Soho outbreak. The cholera epidemic of 1849–1854 was also related to the water supplied by companies in London at the time. The main players were the Southwark and Vauxhall Waterworks Company, and the Lambeth Waterworks Company. Both companies provided water to their customers that was drawn from the River Thames, which was highly contaminated with visible and invisible products and bacteria. Dr Hassall examined the filtered water and found it contained animal hair, among other foul substances. He made the remark that: It will be observed, that the water of the companies of the Surrey Side of London, viz., the Southwark, Vauxhall, and Lambeth, is by far the worst of all those who take their supply from the Thames Other companies, such as the New River Company and Chelsea Waterworks Company, were observed to have better filtered water; few deaths occurred in the neighbourhoods which they supplied. These two companies not only obtained their water from cleaner sources than the Thames, but they filtered the water and treated it until there were no obvious contaminants. As mentioned above, Snow is known for his influence on public health, which arose after his studies of the cholera epidemic. In attempting to figure out who was receiving impure water in each neighbourhood, what is now known as a double-blind experiment fell right into his lap. He describes the conditions of the situation in his essays: In many cases a single house has a supply different from that on either side. Each company supplies both rich and poor, both large houses and small; there is no difference in the condition or occupation of the persons receiving the water of the different companies...As there is no difference whatever either in the houses or the people receiving the supply of the two Water