Karen Weintraub

Special for USA TODAY

A day after a man in a Dallas hospital was diagnosed with Ebola, public health officials said it's likely that more people will come to the United States carrying the deadly virus.

"I would expect that so long as there is such a widespread epidemic in Africa, that even with good screening at the airports, etc., it's possible there will be additional cases," said Jesse L. Goodman, a professor of medicine and infectious disease specialist at Georgetown University Medical Center. "This is a global public health emergency, and I think this indicates that."

The virus, which has sickened at least 7,178 people and killed 3,338 in West Africa, hadn't been diagnosed outside the region until Tuesday.

The man who traveled from Liberia to Dallas on Sept. 19-20 was incubating the disease, but he showed no symptoms while he was traveling — so his fellow travelers are not at risk.

Thomas Frieden, director of the Centers for Disease Control and Prevention, said Tuesday that patients are not contagious until they begin to show symptoms. Ebola is passed through close contact with bodily fluids, such as blood and feces.

That means the family members he was in close contact with from Sept. 24, when he began to show symptoms, through Sept. 28, when he was admitted to Texas Health Presbyterian Hospital, may be at risk.

Frieden said those people are being identified and will be tracked for the next 21 days, taking their temperature twice daily, to make sure they do not develop a fever. If they do, they will be quickly isolated so that if they are developing Ebola, they won't pass it on.

Ebola, unlike the flu virus, does not routinely travel through the air, so people who might have ridden in an elevator with him or eaten at the same restaurant are not at risk.

American hospitals have long been expecting an Ebola case here because of the scope of the outbreak in the West African nations of Liberia, Sierra Leone and Guinea.

There have already been several false alarms when patients with a travel history to West Africa were quarantined when they fell ill, but it turned out they were sick with something other than Ebola.

"Every hospital has plans in place to care for patients with infectious diseases, and hospitals across the nation have been and will continue to update their policies as we learn more," Rich Umbdenstock, president and CEO of the American Hospital Association, said in a prepared statement. The CDC has sent guidance to hospitals on the best practices for coping with Ebola.

Most patients with Ebola start showing symptoms about a week after infection. That long gap between exposure and symptoms makes it more likely that people will be able to board flights and travel to the USA, Goodman said.

Airports in West Africa, where flights have been drastically reduced, are screening departing passengers for fevers, but it's not surprising that they would miss a patient like the man in Dallas, even if they are thorough, Goodman said.

There is no way to detect the virus before those symptoms appear. Frieden said Tuesday that even the gold standard test for Ebola can't detect the virus until symptoms are raging.

Goodman said it may make sense to re-screen passengers from Liberia, Sierra Leone and Guinea as they board connecting flights. Most travelers from West Africa must stop elsewhere in Africa or in Europe before traveling to the United States.

"We can't be vigilant enough about this, or be overconfident," Goodman said. "Everybody needs to stay on top of this."