Transcript for Health Officials Retrace Texas Ebola Patient's Movements in Dallas
A major American city is on high alert tonight after the first patient was diagnosed in the U.S. With ebola. Doctors are scrambling to find and monitor everybody this patient contacted, including children in his own family. And there are new and urgent questions tonight about why he was initially sent home from the hospital, even after he told a nurse that he had travels from Africa's hot zone. ABC's Cecilia Vega is outside that hospital tonight, as we investigate ebola in America. Reporter: Tonight, the city of dallases is on high alert. Inside this hospital, a worst fear realized. Ebola is in America. This is all hands on deck. Reporter: The patient's name, Thomas Eric Duncan. An african man who traveled to the U.S. From Liberia. Tonight, in serious condition and under extreme quarantine. A team of CDC detectives streaming into Dallas, hoping to keep this contagion completely contained. Trying to piece together his moves. As questions swirl about this local hospital, where the six man was allowed to leave the E.R., even after reporting he'd visited a stricken country in Africa. Now, a community in fear. Right here in Dallas. And that's terrifying. Reporter: One of the deadliest viruses on the planet, right in their backyard. We're outside the apartment complex in north Dallas where the patient first came right after he arrived in the united States. Health officials telling us today that somewhere inside this apartment complex is the man's family right now. They have been told not to leave. There are adults and there are children. While health officials monitor their well being for the next 21 days, no school, no work. They can not leave this building. I am scared because there's a lot of children out here and, you know, a lot of us, this is something new to us and it's something scary that I never thought would come here in the United States. Reporter: It all began on the 19th of September, day one. Already infected, but no symptoms, Duncan leaves Liberia. At the airport, his temperature probably taken with a thermometer like this. But no fever yet. So, he boards a plane. Flying to Brussels, hen getting on a united airlines flight full of passengers that flew through dulles. The next day, arriving in Dallas. Heading to this apartment complex to visit family. Including several children. Still, no signs of ebola. But four days later, he starts getting sick. And that's when his ebola becomes contagious to others. Just over 24 hours later, he's so ill, he walks into this Dallas emergency room, telling a nurse he'd travels from west Africa. But here, the critical breakdown. That nurse didn't pass the information along and he's sent home. A checklist was in place for ebola in this hospital for several weeks. Regretfully, that information was not fully communicated throughout the full team. Reporter: Two days later, Duncan rushed back to the hospital. This time, by ambulance. And on Tuesday, he tests positive for the ebola virus. The first person ever diagnosed on American soil. Today, a race to track down those who have come into contact with the infected man. Investigators monitoring up to 18 people from the three emts and doctors and nurses who treated him to family members, including those five children. Students at these Dallas schools. I know that parents are being extremely concerned about that development. But let me assure these children have been identified and they are being monitored. Reporter: Ebola spreads through bodily fluids like sweat and blood. But only those showing symptoms can pass on the virus to others. Once infected, the virus can take up to 21 days to start causing symptoms, like fever and vomiting. Arepeople are the people of Dallas, the people of Texas, here in the U.S., are we safe from ebola? I have all confidence in the CDC that we have contained this virus within the number of people that we're looking at now. Reporter: As of tonight, none of those family members or health care workers who came into close contact with the patient are showing any symptoms. And now the burning question. What went wrong here? And how can we prevent another person from bringing this deadly virus to our shores? Dr. Richard Besser is reporting from the center of the outbreak in Liberia. Reporter: It will happen again. It may not be next week or next month. But as long as ebola runs rampant in west Africa, someone else will bring it back to america.besser is at the heart of the battle against ebola. In the same city where the infected man in Dallas may have contracted the virus. Here, there are more patients than doctors can handle. Fears fueling violence and riots. 3,000 people have died in the region. In an attempt to control the spread, each airline passenger is checked for signs of fever at the pair port. But sick passengers aren't always showing symptoms, which is how one of them made it to America undetected. Reporter: The passengers on the plane with the infected man were not at risk. You are only contagious after you start to show symptoms. And officials tell us that he developed symptoms several days after he arrived in America. The only people who are truly at risk here are those who came into close contact with him after he became ill. Reporter: Nancy write boll is one of ebola's lucky survivors. An American missionary who was infected in Africa, then flown to America for treatment. I was running a fever, I had a headache. I was just weak and my muscles ached. Reporter: She knows what it's like to realize you have the dreaded virus. But she also knolls what it's like to be cured. The day that the doctor came in and said, Nancy, the tests are turning around, I just remember saying to the doctor, praise the lord. Praise god. And please know to the family and to him that we are praying for him as you walk through this difficult time. Reporter: A comforting thought tonight for Thomas Duncan. For "Nightline," I'm Cecilia Vega in Dallas.
This transcript has been automatically generated and may not be 100% accurate.