Snow devised and developed the theory that cholera is communicated by means of a contaminated water supply. His essay on the mode of communication of cholera, first published in 1849 with a second edition in 1855 reported his investigations into the effect of the water supply on certain districts of South London in the epidemic of 1854.
‘It is probable also, that the water of the East London Company, obtained above Lea Bridge, had no share in propagating the malady (Cholera).’
‘The water of the East London Company is also free from the contents of sewers, unless it be those from the neighbourhood of Upper Clapton, where there has been very little cholera.’
 Communication of Cholera, pp. 64-5. John Snow. 1855
 Communication of Cholera, p. 9. John Snow. 1855