The first patient to be diagnosed with Ebola in the U.S. initially told a Dallas hospital nurse he had just returned from Liberia but was sent home with antibiotics, hospital officials acknowledged on Wednesday.

When the man — identified by AP as Thomas Eric Duncan — returned to Texas Health Presbyterian’s emergency room last Sunday in worse condition, his diagnosis was confirmed. He is currently in isolation in serious but stable condition.

Now, health officials are observing up to 18 people, including children, who had contact with the patient, Reuters reports.

"We have a seven-person team in Dallas today helping to review that with the family and make sure we identify everyone that could have had contact with him," Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, told the Today show.

Five Dallas-area students who came into contact with the man are being monitored, but "did not have any symptoms and so the odds of them passing on any sort of virus is very low,” according to Dallas Independent School District superintendent Mike Miles.

Texas officials reportedly said health care providers who treated the patient had tested negative so far and there were no other suspected cases in Texas.

"People can be confident here in this country that we have the medical infrastructure in place to prevent the broad spread of Ebola," White House spokesman Josh Earnest said on CNN.

The World Health Organization reports that the Ebola virus is believed to have infected more than 7,100 people in West Africa and has caused more than 3,300 deaths.

It can take as long as 21 days for the symptoms, such as fever, muscle pain, vomiting and bleeding, to appear. The virus does not become contagious until symptoms show.

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