The director of the Centers for Disease Control and Prevention, Thomas Frieden, confirmed on Tuesday the diagnoses of the first case of Ebola in the United States. (AP)

Months after the deadliest Ebola outbreak in history began ravaging West African countries, a man who flew from Liberia to Dallas became the first case of Ebola to be diagnosed in the United States.

Health officials stressed that they are confident they can control this situation and keep the virus from spreading in the U.S.

"We're stopping it in its tracks in this country," Thomas Frieden, director of the Centers for Disease Control and Prevention, declared during a news conference Tuesday afternoon.

The man who is infected, who was not identified, left Liberia on Sept. 19 and arrived in the U.S. the following day to visit family members. Health officials are working to identify everyone who may have been exposed to this man. Frieden said this covered just a "handful" of people, a group that will be watched for three weeks to see if any symptoms emerge.

"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," 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."

[RELATED: Investigating Ebola: How contact tracing will work in Texas.]

There were more than 6,500 reported cases of Ebola in Guinea, Liberia and Sierra Leone as of Tuesday, and the crisis has been blamed for more than 3,000 deaths, according to the World Health Organization. Ebola was first identified in 1976, and the current outbreak in West Africa is considered the largest and most complex in the history of the virus, with more cases and deaths than every other outbreak combined.

Extra help arrives to help contain the Ebola outbreak in West Africa, which has claimed more than 3,300 lives. (Reuters)

Until now, the only known cases of Ebola in the U.S. involved American doctors and aid workers who were infected and returned to the country for treatment. One of them, Richard Sacra, was discharged last week from a Nebraska hospital. Days later, the National Institutes of Health in Bethesda admitted an American physician who was exposed to the Ebola virus in Sierra Leone. There were reports of possible Ebola patients in New York, California, New Mexico and Miami, but all of them tested negative for the virus.

The unidentified person with Ebola is being treated in intensive care at Texas Health Presbyterian Hospital Dallas, according to Edward Goodman, the hospital's epidemiologist.

People who traveled on the same plane as this man are not in danger because he had his temperature checked before the flight and was not symptomatic at the time, Frieden said. Ebola is only contagious if the person has symptoms, and can be spread through bodily fluids or infected animals but not through the air.

"There is zero risk of transmission on the flight," Frieden said.

[RELATED: How do you get Ebola?]

Still, the fact that the disease has been confirmed on American soil immediately sparked fears in the U.S., turning a public health crisis from a faraway news story to something that makes people reach for Purell and facemasks. But experts said it was impossible to imagine that Ebola, which a CDC estimate projects could infect up to half a million people by January, would remain completely outside the country's borders.

“It was inevitable once the outbreak exploded,” said Thomas Geisbert, a professor at the University of Texas Medical Branch at Galveston, who has researched the Ebola virus for decades. “Unless you were going to shut down to shut down airports and keep people from leaving [West Africa], it’s hard to stop somebody from getting on a plane.”

But Geisbert quickly underscored how unlikely the virus is to spread in the United States. For starters, he said, officials placed the sick man in quarantine quickly in order to isolate him from potentially infecting others. In addition, health workers are already contacting and monitoring any other people he might have had contact with in recent days.

Two Dallas Fire-Rescue paramedics and one paramedic intern are being monitored for Ebola symptoms after transporting the patient to the hospital. The three EMS workers will remain at home for 21 days, Dallas Fire-Rescue Lt. Joel Lavender said Tuesday night. Their ambulance was decontaminated after they transported the patient, Lavender said.

“The system that was put in place worked the way it was supposed to work,” Geisbert said.

That doesn't guarantee that no one else will get infected, because the sick person could have transmitted the disease to someone else before being isolated. But that approach almost certainly ensures that the United States will quickly contain the disease.

The deadliest Ebola outbreak in history is centered in the West African countries of Liberia, Sierra Leone and Guinea, though there is a separate outbreak in Congo. Unlike in West Africa, where the affected countries have fragile or barely existent health care systems, where people are being turned away from treatment centers, where family members are caring directly with those sick and dying from Ebola, the U.S. is far more equipped to isolate anyone with the virus and provide the highest level of care.

For months, the CDC has been conducting briefings for hospitals and clinicians about the proper protocol for diagnosing patients suspected of having the virus, as well as the kinds of infection control measures to manage hospitalized patients known or suspected of having the disease. Many procedures involve the same types of infection control that major hospitals are already supposed to have in place.

Early recognition is a critical element of infection control. Symptoms include fever greater than 101.5 degrees Fahrenheit, severe headache, muscle pain, vomiting, diarrhea and contact within 21 days before onset of symptoms with the blood or other bodily fluids or human remains of someone known or suspected of having the disease or travel to an area where transmission is active.

The CDC also has scheduled more training for U.S. workers who either plan on volunteering in West Africa or want to be prepared in the event that cases surface at their own hospitals.

President Obama spoke with Frieden on Tuesday afternoon regarding the way the patient is being isolated and the efforts to scour the man's contacts to seek out any potential other cases, the White House said.

Frieden said during the news conference that the man who is infected did not develop symptoms until about four days after arriving in the country. This man sought medical treatment on Friday, two days after symptoms developed, but was evaluated and released. He was admitted to the hospital on Sunday before being placed into isolation. Frieden, who would not say if the man was a U.S. citizen, said the man is not believed to have been working as part of the response to the Ebola outbreak.

David Lakey, head of the Texas Department of Health Services, said the state's laboratory in Austin, Tex., received a blood sample from the patient on Tuesday morning and confirmed the presence of Ebola several hours later. This laboratory was certified to do Ebola testing last month.

Here is a video showing how the Ebola virus works:

As the worst Ebola outbreak in history unfolds in West Africa, The Post's Joel Achenbach explains how the deadly virus wreaks havoc on the human body. (Davin Coburn/The Washington Post)

And here is more on the spread:

Sarah Larimer, Todd C. Frankel and Lena Sun contributed to this report.

[This post has been updated multiple times.]

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