The patient, who was identified in US media as Thomas Eric Duncan, travelled from Liberia to Texas, where he was diagnosed earlier this week.
Duncan did not have a fever when he departed Liberia on September 19, said Centers for Disease Control and Prevention chief Tom Frieden.
However, he began to feel sick on September 24, and once a patient begins to show symptoms like fever, vomiting and diarrhoea, they can infect others who come in close contact with their bodily fluids.
Duncan was initially sent home when he first sought medical care, leaving a four-day span when he was sick and contagious while in contact with others, sparking concern over how many others may have been exposed.
About 100 people who had potential contact with Duncan are being assessed, said Frieden.
"We have interviewed most of those people, but far from all," Frieden added.
"We have identified a handful of people who may have had exposure," he said, adding that they were "people from the household and healthcare settings as well." Fourteen people have so far tested negative, he said.
United Airlines, which operated two of the flights Duncan took to reach Texas, has also issued a statement with flight details, suggesting any concerned passengers contact the CDC, though the company stressed that officials say there was "zero risk of transmission" at the time.
Two local schools with links to those possibly exposed to Duncan reported high rates of absenteeism as worried parents kept children at home.
"My mom doesn't want to send us until things get cleared up," said one student, Carlos Antunez, who had been withdrawn from his school. "Many of the parents pulled their students out of school."