Mapping in the time of cholera...
Maps played a key role in ending the famous in just a matter of days.
Given the sophisticated technology at our fingertips - it's quite a surprise to learn that had if it happened today, experts say couldn't have done what he did.
I went to Broadwick Street in London's Soho area with Secretary of the John Snow Society, Dr Rosalind Stanwell-Smith, to find out why.