On August 31, 1854, people in Soho began dying of cholera. Whilst cholera outbreaks were not uncommon in 19th century London, this particular epidemic was to change the course of history.

John Snow, a physician investigating the outbreak, interviewed patients and residents to map the disease spread. He pinpointed a high concentration of cases around a water pump on Soho’s Broad Street, which he discovered was drawing water from an area of the Thames contaminated by a cesspit.

Thus the mystery of the Soho cholera outbreak was solved. The Broad Street pump handle was removed and the outbreak quickly came to a close. Snow’s clever detective work saved many lives in 1854, but the impact of his findings still reverberate today.

Snow’s investigations laid the groundwork for the field of epidemiology: the scientific study of disease patterns within populations. Epidemiology comes from Greek words ‘epi’, ‘demos’, and ‘logos’, meaning ‘the study of people’. Epidemiologists track the spread and impact of disease on the population level, drawing on medicine, policy, social sciences and statistics to help piece together an understanding of disease.

Since John Snow’s great epidemiological study of 1854, this field has become instrumental to understanding and controlling the spread of epidemics. During the initial stages of the HIV outbreak in the 1980s, epidemiological investigations helped scientists to identify the mode of transmission and to provide information to an anxious public.

More recently, epidemiology became central to the fight against Ebola in Western Africa. During the outbreak, a team led by Oxford University scientists used epidemiological techniques to create a map predicting where future outbreaks might occur. By studying the movements of people and animals likely to be infected by Ebola, the group were able to identify areas most at risk of infection.

And as tools and techniques advance, epidemiologists are becoming better able to track the spread of disease through communities and geographic locales. Where John Snow had to rely on dot maps and direct interviews, epidemiologists today have microbiology and genome sequencing.

And from the very small comes the very big: understanding a pathogen’s molecular characteristics can help epidemiologists to predict its impact on a village, a region, even on a national and international level.

Using genome sequencing, scientists can track the evolution and spread of pathogens during an outbreak. Because viruses and bacteria evolve quickly, different patients will carry different versions of the same disease.

And so, by sequencing the genetic code of an Ebola strain found in different areas, it is possible to track the movements of the virus across different regions through time. This information can help epidemiologists to predict how quickly and in what region a pathogen might spread, and what steps can be taken to prevent infection.

Back in 2015, a handheld device developed by Oxfordshire Nanopore Technologies was used to sequence the genome of the Ebola virus, allowing scientists to study the viruses mutations in real time.

And scientists from Oxford University recently used genome sequencing to uncover vital new epidemiological information about the Zika virus and its spread. The group travelled 2,000 km across Brazil assessing samples from patients infected with the virus.

Using cutting-edge techniques, the group were able to scrutinise the virus’s genome and track its spread across Brazil. Their research revealed that Zika was present in Brazil for an entire year before the first confirmed cases in May 2015.

Modern epidemiological research is providing unprecedented insights into how diseases emerge, spread and evolve within communities. And whether it’s at the molecular level, the human level, or the international level, the more we know about viruses and bacteria, the better equipped we are to fight disease and save lives.