How does contact tracing stop an Ebola outbreak?

Contact tracing stops Ebola by finding the people a sick person may have infected, watching them during the window when symptoms could appear, and isolating anyone who falls ill before they pass the virus on. It is the process of identifying people who may have been exposed to an infected person, then collecting more data to assess transmission, with the aim of reducing infections in the population.

For Ebola, it involves finding everyone who had close contact with infected individuals and monitoring them for signs of illness for 21 days, which matches the longest incubation period of 2 to 21 days. If any of these contacts comes down with the disease, they should be isolated, tested and treated. Then the process is repeated, tracing the contacts' contacts.

The work is detective work among people who know each other. A study published in December 2014 found that transmission of the Ebola virus occurs principally within families, in hospitals and at funerals, and that the third person in any transmission chain often knew both the first and second person. It concluded that the epidemic would not be as difficult to control as feared if rapid, vigorous contact tracing and quarantines were employed. On 17 December 2014, Sierra Leone launched "Operation Western Area Surge", and workers went door-to-door in the capital looking for possible cases.

Tracing links to the number R. If R is less than 1, an epidemic dies out; above 1 it keeps growing exponentially. Every contact found and isolated before they infect others pulls R down. The same method was used for smallpox, whose eradication was achieved by exhaustive contact tracing to find all infected persons.