Ebola has arrived in the United States — an unwelcome but not entirely unexpected guest.

A major part of the public health response to the case confirmed in Dallas will be a process called contact tracing. Because you can only become infected by Ebola through direct contact with the bodily fluids of someone who has the virus and is exhibiting symptoms, health officials need to identify every single person who has come in contact with the Dallas patient since Wednesday, when he became symptomatic.

It’s unclear exactly how many people the patient may have encountered between then and Sunday, when he was admitted to Texas Health Presbyterian Hospital in Dallas and placed in isolation.

“We are just beginning the process; the investigation has just begun today,” CDC director Thomas Frieden said Tuesday. “But the health department has already been very forward-leaning on that and already has locating information” for those contacts, Frieden said. He characterized them as “a handful” of people, mostly family members.

Two Dallas Fire-Rescue paramedics and one paramedic intern are being monitored for Ebola symptoms after transporting the patient to the hospital. The three EMS workers will remain at home for 21 days, Dallas Fire-Rescue Lt. Joel Lavender said Tuesday night. Their ambulance was decontaminated after they transported the patient, Lavender said.

“Besides the case yesterday, we have not conducted any additional Ebola tests in our state lab, and there are no other suspect cases,” Texas Department of State Health Services spokeswoman Carrie Williams said in an email on Wednesday morning.

Here’s how the CDC describes the contact tracing process.

First, public health workers must identify anyone who has come into contact with a person sick with Ebola. This group is then monitored for a period of 21 days from the last time they came into contact with the Ebola patient, to see if any symptoms emerge.

Symptoms — which start to show up sometime sometime between two to 21 days from exposure — include high fever, severe headaches, diarrhea, vomiting, unexplained bleeding, muscle pain and weakness. Ebola is contracted through exposure to bodily fluids, whether that’s through an infected person or a contaminated object such as a needle.

If a contact becomes sick with a high fever or displays other Ebola symptoms during that 21-day window, he or she is immediately put into isolation and tested for the virus.

This begins a new round of contact tracing, as public health workers have to identify everyone who came into contact with this new potential patient. Just one missed contact in the chain can perpetuate an outbreak.

In Nigeria, one Ebola-infected man who arrived at the Lagos airport in July led to an Ebola outbreak that left eight dead out of 20 probable or confirmed cases of the deadly virus. Contact tracing in that country led to a pool of 894 contacts, with nearly all of them now past the monitoring phase.

At one point, 477 people in one Nigerian city were under surveillance, with response team members visiting homes daily to check for symptoms.

Health workers conducted a total of 18,500 face-to-face visits in Nigeria after Ebola arrived there, and the massive and rapid response by health officials in Africa’s largest country appears to have stopped the outbreak, the CDC said Tuesday.

The task may be more simple in the United States: The Ebola patient in Dallas didn’t start showing symptoms until days after he arrived in Texas. So the people he came into contact with on the flight and in the airport are not going to be a part of the Texas contact-tracing effort.

Public health workers will, however, interview every family member and identify all possible people who came into contact with the patient once he did become sick.

“We outline all the movements that could have occurred from the onset of symptoms,” Frieden said. “We always err on the side of identifying more contacts rather than less.”