How many people?
Health officials say they are investigating about 100 people who might have come in contact with Duncan, who traveled to the United States from Liberia, a West African country that's been ravaged by Ebola. Duncan left Liberia on Sept. 19 and arrived in Dallas on Sept. 20 after a connection at Dulles International Airport near Washington. Earlier this week, officials said 12 to 18 people might have interacted with him after he became symptomatic, including five children who attend four different schools.
What if I was on the plane with Duncan? Or in the airport?
Ebola spreads through the transmission of bodily fluids, like blood or vomit, so simply breathing the same air as Duncan wouldn't lead to an infection. And Ebola patients aren't contagious until symptoms develop. Duncan didn't start showing signs of an infection until days after he arrived in the United States.
"There is zero risk of transmission on the flight," Thomas Frieden, director of the Centers for Disease Control and Prevention, said earlier this week.
Is it troubling that he wasn't hospitalized faster?
A hospital official did say Duncan should have been isolated sooner, when he first sought care at the medical center. (He was given antibiotics and sent home.) Officials are now working to find everyone who encountered Duncan. That's a process called contact tracing.
“We are working from a list of about 100 potential or so possible contacts and will soon have an official contact tracing number that will be lower,” Texas Department of State Health Services spokeswoman Carrie Williams said in a statement. “Out of an abundance of caution, we’re starting with this very wide net, including people who have had even brief encounters with the patient or the patient’s home. The number will drop as we focus in on those whose contact may represent a potential risk of infection.”
This video explains Ebola and how it spreads.
A person can become sick with Ebola if he or she is exposed to contaminated objects, like a needle or a syringe, or by being exposed to the broken skin of an infected patient. Even funerals can be dangerous.
The World Health Organization says Ebola can also be spread via "surfaces and materials (e.g. bedding, clothing)" that are contaminated with the bodily fluids of infected people.
"Of course we made the rule, so I am home for 21 days," Dahn told the AP. "I did it on my own. I told my office staff to stay at home for the 21 days. That’s what we need to do."
Here's a video explaining Ebola — and what happens to patients when they fall ill — in more detail:
The CDC says people who care for Ebola patients are at high risk, for obvious reasons. (They're in close contact with patients suffering from it.) Health-care workers on the front lines of the Ebola outbreak often look like this:
"The bottom line here is that I have no doubt that we will control this importation, or this case of Ebola, so that it does not spread widely in this country," the CDC's Frieden said. "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."
[This post has been updated.]