In June 1866, a localized epidemic in the East End of London claimed 5,596 lives, just as the city was completing construction of its major sewage and water treatment systems; the East End section was not quite complete. It was also caused by the city's overcrowding in the East End, which helped the disease to spread more quickly in the area. Epidemiologist William Farr identified the East London Water Company as the source of the contamination. Farr made use of prior work by John Snow and others, pointing to contaminated drinking water as the likely cause of cholera in an 1854 outbreak. In the same year, the use of contaminated canal water in local water works caused a minor outbreak at Ystalyfera in South Wales. Workers associated with the company, and their families, were most affected, and 119 died.