Contact tracing: How to stop Ebola in its tracks

Although health officials have not said how many people will be involved in contact tracing in this case, the C.D.C. director, Dr. Thomas R. Frieden, has said that his team is focused on identifying every person who had contact with Thomas E. Duncan, who arrived in Dallas from Liberia on Sept. 20.

Ebola is transmitted through direct contact with infected bodily fluids, and someone who has it is contagious only after symptoms begin — in Mr. Duncan’s case, on his fourth day in the United States. From that moment until he was put into isolated care, as many as 100 people may have been exposed to the virus, health officials in Texas said Thursday. It is the job of contact tracers to find every one of them.

Although Ebola is new to the United States, the goal of contact tracing is the same in any disease: Track down those who could have been exposed, interview them and monitor them — in this case, for 21 days, the incubation period of the Ebola virus. Those showing symptoms or thought to be at particular risk are isolated, and their contacts traced and monitored, until the outbreak is declared dead.