Despite national guidelines for evaluating potential Ebola cases and worldwide awareness of an epidemic that has killed thousands in West Africa, a man who later tested positive for the disease was released from a Dallas hospital even though he said he had traveled to Texas from Liberia.

The man returned two days later and was placed in isolation before he became the first person ever diagnosed with Ebola in the United States. But as a result of this lapse, Thomas Duncan was able to leave the hospital and interact with more people while experiencing symptoms, which is when the virus becomes contagious.

Now officials are watching at least 12 to 18 people with whom the man may have come into contact, including five children he may have been around during the time between his hospital visits, officials said Wednesday.

There are no other suspected or confirmed cases of Ebola in Texas, and authorities continue to stress that they can contain the virus and prevent a rampant spread.

The man told a nurse on Friday 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 treating the man.

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

As a result, 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 that day, Lester said.

The oversight allowed this man to leave the hospital at a time when he was symptomatic, according to the timeline outlined by federal health officials. The Centers for Disease Control 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.

The Texas Ebola patient is named Thomas Eric Duncan, according to the Associated Press. Duncan’s sister told the AP he was sent home with antibiotics that day.

Duncan returned to the same hospital in Dallas two days later 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 went to the hospital on Friday, 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. He had a fever and some abdominal pain, Goodman said.

Health officials are still working to track down anyone who may have come into contact with Duncan. The CDC has sent a team of 10 people to Texas to work with state and local departments at tracking anyone who had close contact with this patient.

Dallas County health officials says they are continuing to monitor the patient’s family members and are checking symptoms twice each day. The state health department said they have no other suspected cases at this point and said they have not conducted any other Ebola tests so far.

“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.”

Five of the people who came into contact with Duncan were “school-aged children,” Texas Gov. Rick Perry (R) said during the same news conference. These children are being watched at home for any signs of the illness.

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 the Ebola patient.

None of these 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.

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

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

Duncan was identified Wednesday by the Associated Press and Liberian health officials, according to the New York Times.

Henry N. Brunson, general manager of Safeway Cargo in Monrovia, Liberia, told The Post that 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 Ministry of Health, 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 little about his driver, a man he said was in his mid-30s and whom he described as competent but occasionally “arrogant.” 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.

The State Department has warned U.S. citizens against non-essential 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.

United Airlines said on Wednesday that it was told by the CDC that the man with Ebola had used the airline to travel from Brussels to Dulles International Airport outside Washington, D.C., before flying from Dulles to Dallas-Fort Worth. However, authorities have said that there is no danger of anyone getting sick from the flight because the man was not symptomatic at the time.

On Aug. 1, the CDC issued national guidelines for evaluating potential Ebola cases. The advice boiled down to a three-prong test: If a patient had a temperature of at least 101.5 degrees, plus another symptom such as headache, body pains or diarrhea and had visited a West African nation hit by the Ebola outbreak in the previous three weeks, that’s a red flag for Ebola.

Hospitals were told to first rule out malaria, a disease with similar symptoms. And several of the suspected Ebola cases that popped up over the summer in hospitals in Europe and the United States turned out to be that mosquito-borne disease.

Later in August, the CDC helped 13 states develop the capability to test for Ebola virus in state public health labs. Texas was one of those states. That way blood samples would not need to be shipped to the CDC’s lab in Atlanta.

And in early September, the CDC issued more detailed steps for 911 systems and first responders facing potential cases of Ebola, such as suggesting 911 dispatchers ask people calling for help if they’ve traveled recently to West Africa. But that advice was aimed only at places where the Ebola risk is “elevated in a community,” according to the CDC. That didn’t apply to any U.S. communities four weeks back, though it does apply now to Dallas.

The CDC’s recommendations for hospitals were re-emphasized Tuesday by Dallas County Health and Human Services, which urged hospitals to review checklists for evaluating potential Ebola cases.

But it’s unclear whether that message is reaching front-line health workers. National Nurses United, a union representing 185,000 nurses nationwide, said it found that 60 percent of its members felt U.S. hospitals were not prepared for Ebola.

“If hospitals have been told how to respond,” said the group’s executive director RoseAnn DeMoro, “they haven’t told the people taking care of the patients. They are outside the information loop.”

Officials again outlined how difficult it is to contract Ebola, which is spread only through direct contact and bodily fluids. They also again stressed that the health care system in the U.S. is prepared to contain this virus.

“This is not West Africa,” Lakey said. “This is a very sophisticated city, a very sophisticated hospital. The dynamics are so significantly different than they are in West Africa that the chances of it being spread are very, very, very small.”

Todd C. Frankel, Leonard M. Bernstein and Brady Dennis contributed to this report.

[This post has been updated. First published: 1:55 p.m. Last updated: 5:45 p.m.]