Detail from John Snow's map of the Soho cholera outbreak (1855).
Though relatively unremarkable by today's standards, this map was a huge breakthrough for the medical community.
By mapping cholera, Snow revealed that it was radiating out from a single point in the neighbourhood. This point, on Broad Street, was revealed to be a pump. The pump was found to be contaminated with sewage.
This ended the then prevailing theory that cholera was spread by bad air and enabled better methods of preventing it.