The Victorian era saw Britain's population increase rapidly. Industrialisation brought people into towns and cities. Overcrowding, poor sanitation and poverty meant diseases like cholera and typhoid spread easily.

Florence Nightingale was an advocate of the miasma theory, which was that diseases were caused by "bad air". The air or ‘miasma’ was foul smelling and filled with particles of decomposed matter that could cause illnesses. It was thought that the miasma could be eliminated by cleaning and ensuring good ventilation. During the cholera outbreak in 1854, Florence volunteered to nurse patients from Soho, London, at the Middlesex Hospital. The physician John Snow, who was sceptical of the miasma theory, was convinced that cholera was a water-borne disease. He provided conclusive proof by mapping out the cases in Soho, pinpointing one, central, contaminated well. The outbreak subsided soon after the well’s pump handle was removed.

However, the miasma theory prevailed and the problem with sanitation in London came to a crisis with the 'Great Stink' of London in 1858. The overpowering smell from the Thames was so bad that the curtains of the House of Commons were soaked in chloride of lime to try to make it bearable of the MPs. A bill was rushed through Parliament to provide more money to construct a massive new sewer for London, and to build the Embankment along the Thames in order to improve the flow of water and traffic.

Even though the miasma theory was disproved, it made the connection between dirtiness and diseases, encouraging cleanliness and bringing about public health reforms. The miasma theory understood that disease was associated with poor sanitation and that sanitary improvements reduced disease. Because of the miasma theory, scientists investigated decaying matter, which led eventually to the identification of microbes as agents of infectious disease.