The Centers for Disease Control and Prevention is working to determine if a man hospitalized in Dallas with the first confirmed case of Ebola diagnosed in the United States transmitted the deadly virus to others — a process called "contact tracing."
"Contact tracing is a core public health function. We do it in a very systematic manner," CDC Director Tom Frieden said this week. "It's what we do day in and day out and what we will be doing here to ensure there aren't further chains of transmission."
So how exactly does contact tracing work?
According to the CDC and other health officials, this is what happens:
— Interviewing the infected patient, if possible, and the patient's family members;
— Outlining all of the movements that could have occurred from the onset of symptoms until isolation;
— Developing a list of anyone the infected patient came into contact with when he was exhibiting symptoms;
— Interviewing those contacts to establish a map with concentric circles indicating the level of exposure in each case (high, low or intermediate);
— And monitoring those contacts for 21 days.
"We always err on the side of contacting more contacts than less," Frieden said. "Then, in a cascading manner, we identify every other individual who can add to that information."
Thursday morning the Texas Department of State Health Services said investigators are tracking about 100 potential or possible contacts.
"Out of an abundance of caution, we're starting with this very wide net, including people who have had even brief encounters with the patient or the patient's home," Carrie Williams, state health department spokesperson, said in a written statement. "The number will drop as we focus in on those whose contact may represent a potential risk of infection."
If a contact shows symptoms of Ebola — high fever, severe headaches, muscle pain, weakness, diarrhea, vomiting, abdominal pain or unexplained bleeding — he or she is immediately isolated, tested and asked about their contacts, who are also monitored for 21 days. The cycle is repeated until there are no new potential Ebola patients. "Even one missed contact can keep the outbreak going," the CDC warns on its website.
"It's like an ember in a forest fire," Dr. Sanjay Gupta, CNN's chief medical correspondent, told Yahoo News on Wednesday night.
In this case, the patient, Thomas Eric Duncan, a 42-year-old Liberian man, arrived in Dallas on Sept. 20. On Sept. 26, he told a nurse at Texas Health Presbyterian Hospital that he first began experiencing Ebola symptoms two days earlier, but a communication breakdown between the nurse and doctors at the hospital resulted in Duncan being sent home with antibiotics. He returned in an ambulance on Sept. 28, when he was admitted to the isolation unit in critical condition. Duncan was upgraded to serious but stable condition on Wednesday.
That means anyone who came into contact with Duncan on or after Sept. 24 needs to be monitored, including the paramedics who transported Duncan to the hospital. Two Dallas Fire-Rescue paramedics and one paramedic intern are being monitored and will remain at home for 21 days, Dallas Fire-Rescue Lt. Joel Lavender said on Tuesday night.
Frieden said the health department was "forward-leaning" and had been developing the contact list before Duncan's diagnosis.
But according to the Frieden, the list will not include fellow passengers on Duncan's flight from West Africa to the United States, because he did not show symptoms before or during the flight.
There was "zero risk of transmission” to other passengers, he said. According to the CDC, Ebola cannot be spread until symptoms appear and can be spread only through bodily fluids.
"Ebola doesn't spread before someone gets sick, and he didn't get sick until four days after he got off the airplane."
On Wednesday, Texas Gov. Rick Perry said five school students apparently had contact with Duncan over the weekend. The students, who attend four separate schools in the Dallas area, returned to school but did not show symptoms of Ebola. But since Duncan's diagnosis, they are being held out of school out of an abundance of caution, Texas school officials said.
In West Africa — where the World Health Organization estimates the Ebola outbreak has resulted in a total of 6,574 Ebola cases and 3,091 deaths since March — contract tracing is more complicated in a war-torn region inhabited by people often mistrusting of doctors.
Last month, the Washington Post reported that more than 477 people were being monitored for Ebola in Port Harcourt in Nigeria after the death of a doctor there. The first known case of Ebola, from a Liberian-American man who became sick on a flight to Lagos in July, resulted in more than 18,500 face-to-face visits from health workers and 894 people under surveillance, the CDC said on Tuesday. All 894 had exited the monitoring phase without showing signs of the virus.
On Tuesday, Frieden characterized the number of those Duncan might have come into contact with as "a handful" — mostly family members — but one source close to the CDC told Yahoo News that as many as 18 people who might have come into contact with Duncan during that time are being monitored.
“It is certainly possible that someone who had contact with this individual could develop Ebola in the coming weeks," Frieden said. "But there is no doubt in my mind that we will stop it here."
One reason for the CDC's confidence, Gupta said, is that very few Ebola patients are in a position to infect others when they're experiencing symptoms, shrinking the contact list significantly.
"With this infection, you're pretty sick," Gupta said. "You're either in bed or in the hospital. It's not like you're out walking through the mall. So it makes it a little easier to track."