John Snow pioneered the use of epidemiology to trace a cholera outbreak in London in 1854.
Snow mapped cholera deaths around the Broad Street water pump in Soho and noticed that cases clustered among people using that water source. He persuaded local officials to remove the pump handle, although the outbreak was already declining by then.
His analysis challenged the prevailing miasma theory, which attributed disease mainly to poisonous air. Snow argued that contaminated water was spreading cholera, a conclusion later supported by germ theory and knowledge of Vibrio cholerae.
John Snow was a male physician, so this question does not satisfy a “Famous Female Scientists” topic. It should be excluded from the requested batch despite being a valid general-science question.