The first patient to be diagnosed with Ebola outside of Africa during the ongoing epidemic is being treated at a Dallas hospital. Federal and state health officials say the patient traveled from Liberia on Sept. 19. Here's what we know about him and the risk to Americans:

Where did he come from?

Thomas Eric Duncan came from Liberia. He left Monrovia Sept. 19 and arrived in the United States Sept 20. Health officials won’t give many details about the patient, but Centers for Disease Control and Prevention director Dr. Thomas Frieden says he was visiting relatives who live in the United States. That might imply he’s not a U.S. citizen himself. Health officials also say he does not appear to have been a health worker, although they are double checking. The four Ebola patients evacuated to the U.S. for treatment have all been American doctors or medical missionaries.

Can he have infected anybody else?

Frieden says it’s possible, although not very many people would be at risk. “It is certainly possible that someone who had contact with this individual … could develop Ebola in the coming weeks. But there is no doubt in my mind that we will stop it here," Frieden said.

Duncan first felt sick Sept. 24 and went to the hospital in Dallas. But staff didn't suspect Ebola then, so he went home. He came back by ambulance on Sept. 28. He's now critically ill in isolation in the hospital's intensive care unit.

Frieden has been clear that no one on the flight to the United States would have been at risk because the patient wasn’t sick yet when he flew. The patient had been staying with family and not at a hotel, and in the affected countries in West Africa, family members, caregivers and health care workers who tend to patients have the highest risk of infection.

“We have identified all the people who could have had contact with the patient while he was infectious,” Frieden said. “It is only someone who is sick with Ebola who can spread the disease.”

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And Ebola only spreads in bodily fluids, not in the air. “Ebola is a virus. It’s a virus that is easy to kill by washing your hands. It’s easy to stop by using gloves and barrier precautions,” Frieden said. “The issue is not that Ebola is highly infectious. The issue with Ebola is that the stakes are so high. People are infectious with Ebola when they are sick.”

How did he get infected?

CDC and Texas health officials are not giving details about Duncan. But he would have to have been in close contact with someone who was sick with or died from Ebola in Liberia. Ebola spreads in bodily fluids such as blood, vomit or diarrhea. And those fluids have to get inside your body through the mouth, eyes or nose, or in a cut or via a needle stick. The virus doesn’t spread in the air and it cannot persist on surfaces like flu viruses can.

How long until we're sure it's not spreading?

Anyone who might have had direct contact with the patient after he started feeling sick — family members and health care workers, for example — will be watched for 21 days to make sure they don’t start to develop a fever or other symptoms. If they do, they’ll be immediately isolated and tested to make sure they don’t infect anyone else. People at highest risk will be asked to take their temperatures at least once a day.

The Dallas mayor's office says the EMS crew members who transported the patient have been quarantined, just to be safe.

People infected with Ebola usually start to show symptoms within about eight days, but the longest known incubation period is 21 days. No one is known to have developed Ebola infection after that long a time.

Ebola’s been around for decades and doctors understand it pretty well. People do not infect others after they get better, and they do not seem to always be particularly infectious early on in the disease — when they first get a fever, for example. The virus has to build up in the body. The most infectious people are those just about to die or who have just died, which is why burying Ebola victims is especially dangerous.

Frieden said Duncan left Liberia on an overnight flight Sept. 19. Frieden says there's no chance he could have infected anybody on the flight because he would have been checked for fever before leaving Liberia and also when he arrived in the United States.

Can we be sure this won’t set off an epidemic in the U.S.?

Doctors are confident that Ebola cannot spread far in the U.S. People with Ebola get very sick very fast and it’s difficult to conceal, so they will almost certainly seek medical care. Hospitals and clinics know to look for the symptoms and staffers will take the right precautions right away — isolating the patient, wearing gloves, gown and mask and disposing of any potentially infected materials properly.

“We are stopping it in its tracks in this country,” Frieden said.

Ebola is spreading rapidly in Liberia, Sierra Leone and to a lesser degree in Guinea because people are not always following instructions for stopping the virus. People are also unable to get care because there is not enough space to isolate and treat them. Many infections have been linked to funeral practices or to people being forced to care for sick loved ones without any equipment to protect themselves, even basics such as soap and water.

In addition, CDC notes that an outbreak in Nigeria started by a Liberian-American man who traveled after he got sick has now been stopped. “During the outbreak, there were 19 laboratory-confirmed and one probable Ebola cases in two Nigerian states. Nearly 900 patient contacts were identified and followed; all but three have completed 21 days of follow-up period without Ebola symptoms,” CDC says.

“There have been no new cases since August 31 and the last three patient contacts will exit their 21-day follow-up on October 2 — strongly suggesting the outbreak in Nigeria has been contained.” The man, Patrick Sawyer, did not infect anyone else on the two flights he took.