In the 1800s cholera was a mysterious disease killing millions around the world. No-one knew how to stop it till an English doctor, John Snow, began investigating the outbreak of 1854. At a time before germ theory was properly understood, many public health experts thought disease was carried on what they called "bad air". John Snow was alone in thinking cholera was spread through contaminated water and by the time of his death - in 1858 - his theories had still not been fully accepted. Claire Bowes spoke to Dr Nigel Paneth, a biographer of John Snow, about the skills he brought to the developing science of epidemiology.
Photo: Portrait of John Snow (Science Photo Library BBC)