Given this paragraph about London, give me one reason why epidemics were spread in London.

With the onset of the Industrial Revolution in Britain, an unprecedented growth in urbanisation took place, and the number of High Streets (the primary street for retail in Britain) rapidly grew.[91][92] London was the world's largest city from about 1831 to 1925,[93] with a population density of 325 per hectare.[94] In addition to the growing number of stores selling goods such as Harding, Howell & Co. on Pall Mall—a contender for the first department store—the streets had scores of street sellers loudly advertising their goods and services.[91] London's overcrowded conditions led to cholera epidemics,[95] claiming 14,000 lives in 1848, and 6,000 in 1866.[96] Rising traffic congestion led to the creation of the world's first local urban rail network. The Metropolitan Board of Works oversaw infrastructure expansion in the capital and some surrounding counties; it was abolished in 1889 when the London County Council was created out of county areas surrounding the capital.[97]

London's overcrowded conditions is one reason why epidemics spread in London.