Contact tracing, that is determining who may have an infection and then determining who may come into contact with this person, is likely a more recent practice, but we have some historical evidence of partial attempts at least. The earliest recorded evidence comes from the Medieval period, during the Black Death plague, when individuals who were known to have the plague were not only quarantined but their homes were marked. Often a cross or marker would be put so that everyone would be informed that a given house should be avoided.
This would try to help those around an area where someone became infected to know they should avoid contact. This, of course, would have created problems for the infected individual, often sealing their fate, but it could have limited the spread of the plague to an extent. The main problem was there is no evidence any systematic mapping of infected individuals was practiced. One of the earliest efforts to map an outbreak of infection comes from John Snow, who helped track and map the source of a cholera outbreak in London in 1854 (Figure 1). His work discounted the idea that 'foul air' spread cholera and other infectious diseases, as mapping infected individuals demonstrated a district in Soho London, traced to a particular water pump on Broad street, was the source of the outbreak. Simply talking to people, finding out when and where incidents of infection occurred, and then mapping that data helped to create among the first maps of a disease outbreak. This work made him one of the key founders of epidemiology and public health. Soon after these outbreaks, London's sanitation improved, with the pump replaced, with the work also forming the basis of public health that helped to diminish future outbreaks.<ref>For more on John Snow's work, see: Hempel, Sandra. <i>The Medical Detective: John Snow, Cholera and the Mystery of the Broad Street Pump</i>. London: Granta Books, 2007.</ref>
While cholera was a major concern, other outbreaks of infectious diseases were shown to have a given source and spread. Contact tracing, whereby individuals infected were mapped so that the source was identified and isolated, was applied to other infectious disease as the late 19th century progressed. Tuberculosis (TB) was one of the biggest concerns during the rapid urbanization in the late 19th century. Cramped conditions and droplets made the spread of this disease rapid in the growing cities of Europe and North America. By the 1880s, mandatory reporting of TB was required so that public health officials could map and trace the location of outbreaks. This also then created an alert system for public health officials to warn people in affected areas that an outbreak was occurring. While response time was still relatively
, slow, which led to outbreaks not being effectively contained, the fact that public officials now more commonly mapped outbreaks allowed some time for at least some areas to better prepare against outbreaks and infected area to be isolated. By the 1880s, contact tracing was effectively developed.<ref>For more on how contact tracing was used in the 19th century and TB, see: National Collaborating Centre for Chronic Conditions (Great Britain), and Royal College of Physicians of London. <i>Tuberculosis: Clinical Diagnosis and Management of Tuberculosis, and Measures for Its Prevention and Control</i>., 2006. http://www.ncbi.nlm.nih.gov/books/NBK45802/. </ref>
[[File:4-Figure2-1.png|thumb|Figure 1. John Snow's results in a cholera outbreak in London helped to pin the outbreak to a specific pump. ]]