The Ebola outbreak in West Africa has reached the United States, as officials confirm one case in Dallas. Here's how U.S. health officials plan to stop the virus. (Gillian Brockell/The Washington Post)

An ill man who later tested positive for Ebola was not admitted to a Dallas hospital despite saying that he had traveled to Texas from Liberia, allowing him to continue interacting with people before he returned days later and was placed in isolation.

Dallas County health officials say they are continuing to monitor the patient’s family members and are checking for symptoms twice each day. The state health department said it has no other suspected cases­ at this point.

“We’ve been on the lookout for this,” said David Lakey, commissioner of the Texas Department of State Health Services. “Obviously, this disease is scary to a large number of individuals.”

The man, a Liberian, was identified by friends as Thomas Eric Duncan. He is the first person to be diagnosed with Ebola in the United States and was in serious condition Wednesday, officials said. Public health officials continue to stress that they can contain the virus and prevent a rampant spread.

Citing “an abundance of caution,” Texas and Dallas County health officials Wednesday night ordered Duncan’s four close family members to remain home and not see any visitors to prevent possible spread of the disease.

The order was hand-delivered to the family at the Ivy Apartments, in northeast Dallas; it legally requires them to stay home until at least Oct. 19. “The order is in place until the incubation period has passed and the family is no longer at risk of having the disease,” a press release from Dallas health officials said.

The family must also provide blood samples, agree to any other testing and also report any symptoms to Dallas County Health and Human Services immediately, including any fever above 100.5 degrees, headache, nausea, diarrhea or abdominal pain.

“We have tried and true protocols to protect the public and stop the spread of this disease,” said Dr. David Lakey, Texas health commissioner, in the press release. “This order gives us the ability to monitor the situation in the most meticulous way.”

Authorities initially said Duncan interacted with 12 to 18 people, but on Thursday they said that there are potentially 80 people who had contact with him.

Five were “school-aged children,” Texas Gov. Rick Perry (R) said at a news conference Wednesday. These children are being watched at home for any signs of the illness.

Health officials are working to track down anyone who may have come into contact with Duncan after he began showing symptoms. The Centers for Disease Control and Prevention has sent a team of 10 people to Texas to work with state and local departments to track anyone who had close contact with him.

A friend of the family said there was a cookout “for a lot of people” on Sept. 20 that Duncan attended. “That’s what we do when someone comes. We had a welcoming for him,” the friend said, who asked not to be identified.

Aaron Yah, 40, said that he saw Duncan on Sept. 20, the day he arrived in Dallas, and that he seemed fine. “The last time I saw him was Sunday at the Ivy Apartments,” he said. “He was lying down in bed. He told me he was sick. He said he was weak and had diarrhea.”

Duncan traveled to the United States on Sept. 20 on United Airlines Flight 951 from Brussels, changing at Dulles International Airport to Flight 822 to Dallas, according to the airline. United said in a statement that there was no risk to any passenger because the patient was not symptomatic until several days after his trip.

Last Thursday night, the man told a nurse that he had traveled from Liberia to Texas, but this detail was not shared with everyone treating him, said Mark C. Lester, executive vice president of the health-care system that includes Texas Health Presbyterian Hospital, the Dallas facility that is treating him

“Regretfully, that information was not fully communicated throughout the full team,” Lester said at the news conference. “As a result, the full import of that information wasn’t factored into the clinical decision-making.”

Ultimately, the man, who had come to the hospital with a fever and some abdominal pain, was diagnosed with a “low-grade, common viral disease” and sent home, Lester said. Duncan’s sister told the Associated Press that he was given antibiotics.

The oversight allowed him to leave the hospital at a time when he was symptomatic, according to the timeline outlined by federal health officials.

The CDC issued national guidelines in August for evaluating potential Ebola cases, which included seeing if a patient with certain symptoms had recently visited a West African country hit by the Ebola outbreak. And the CDC has been conducting briefings for months to inform hospitals and clinicians about the proper protocol for diagnosing patients suspected of having the virus as well as the ways to manage people suspected of having it.

Duncan returned to the same hospital in Dallas on Sunday in worse shape and was placed in isolation after being recognized as being a potential Ebola patient. On Tuesday, testing at a state laboratory in Austin confirmed that he had the disease.

When Duncan first went to the hospital, he was not vomiting and did not have any diarrhea, so it was unlikely any health-care workers were exposed to the virus, said David Goodman, the hospital epidemiologist.

Authorities are taking every step to monitor Duncan’s health as well as to combat any potential spread, Perry said.

“This case is serious,” Perry said. “Rest assured, our system is working as it should.”

The Dallas Independent School District said it was told Wednesday morning that five students who attend four of its schools — one high school, one middle school and two elementary schools — may have had contact with Duncan.

None of those students are showing any symptoms, so there is no indication that the disease could have spread to any other students or staff, the district said.

In at least two of the schools, hundreds of students received notices Wednesday afternoon alerting parents that “one of our students may have had contact with an individual who was recently diagnosed with the Ebola virus.” The notices, signed by the principals at Emmett J. Conrad High School and Sam Tasby Middle School, said the exposed students had been advised to stay home from school and were under close observation by the Dallas County Health and Human Services Department.

Additional nurses were being sent to both schools as a precaution and extra custodial services were arranged for thorough cleaning of both buildings each evening.

Police kept the news media off school premises as well as the grounds of the Ivy Apartments, where it is believed Duncan spent several days with relatives before being hospitalized. TV cameras and microphones were set up behind the closed iron gates that surround the mustard-colored apartment complex, whose owners refused to let reporters interview residents on the property.

Henry N. Brunson, general manager of Safeway Cargo in Monrovia, Liberia, said Duncan was his personal driver for 14 months until Sept. 4, when Duncan left the keys to the car on a desk in the company’s office about 3 p.m., as he typically did when leaving for the day. But Duncan never returned, and Brunson never heard from him again.

Then his former chauffeur turned up on CNN, and Brunson heard from the Health Ministry, confirming that Duncan was sick with Ebola in Dallas.

“He didn’t inform me he was quitting the job,” Brunson said. “He walked away from the job September 4th.”

Brunson said he knew nothing about Duncan’s previous jobs or his personal life, other than that he was widely rumored to have a son in the United States.

Duncan showed no symptoms of Ebola infection as of his last day with the company, Brunson said, so he and his office staff are not concerned that they will come down with the deadly disease. Ebola is communicable through direct contact with an infected person’s body fluids, and only when he or she has symptoms such as fever, vomiting or diarrhea.

More than 3,000 people have died in the current outbreak, which is the worst in history. The State Department has warned U.S. citizens against nonessential travel to Liberia and Sierra Leone. There are currently no plans to alter the travel warning in the wake of Duncan’s diagnosis, according to a State Department official.

On Aug. 1, the CDC issued national guidelines for evaluating potential Ebola cases that included seeing whether a patient had symptoms and had been to a West African nation recently. Hospitals were told to rule out malaria, which has similar symptoms. And last month, the CDC issued more detailed steps aimed at first responders and 911 dispatchers facing potential Ebola cases.

Late Tuesday evening, officials from agencies including the CDC, the Food and Drug Administration and the National Institutes of Health held a conference call to figure out how best to handle people identified as having contact with Duncan. The consensus was that they should simply watch those people constantly for the next several weeks, ready to isolate anyone at the first sign of symptoms, said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases.

That capacity to do rigorous “contact tracing” in order to halt the spread of the disease, Fauci said, “is the entire core of everything. That’s the difference between ourselves and a place like Liberia.”

It’s why there will be no serious outbreak here, he said, adding, “It’s conceivable that one of the contacts of this person is going to wind up getting Ebola.” But officials would quickly stop it from spreading further.

“When somebody tells you that they’re from West Africa, that’s a big red flag,” he said, but added, “That happens. It’s human nature. People make mistakes.”

Nutt reported from Dallas. Todd C. Frankel, Lenny Bernstein, Abby Phillip, Ashley Halsey III and Brady Dennis contributed to this report.