The Obama administration is considering imposing a quarantine on health workers returning from Ebola-afflicted countries in West Africa after a New York City doctor who had worked in that region tested positive for the often-deadly disease.

A federal Centers for Disease Control spokesman said it is like that changes will be made to current CDC guidelines for medical workers returning from the nations of Guinea, Liberia and Sierra Leone, Reuters reported Friday afternoon.

"There are lots of options being discussed," the spokesman said, according to the Reuters news service.

"We want to strike the right balance of doing what is best to protect the public's health while not impeding whatsoever our ability to combat the epidemic in West Africa. Our risk here will not be zero until we stop the epidemic there."

But soon after that announcement, the governors of New York and New Jersey jointly revealed that they have already instituted a policy of mandatory quarantines of up to 21 days for medical workers returning from the three West African countries after treating Ebola patients there, and also are implementing additional screening protocols for Ebola at JFK and Newark Liberty Airports.

New Jersey Gov. Chris Christie said authorities will quarantine a woman stopped Friday at Newark's airport after officials learned she had treated Ebola patients in West Africa, and was headed to New York. The woman is not showing any symptoms, he said.

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White House spokesman Josh Earnest earlier was repeatedly pressed with questions about why Dr. Craig Spencer was not in quarantine after he returned to New York on Oct. 17 on the heels of treating Ebola patients in Guinea.

Spencer, 33, instead was able to freely throughout New York until he developed a fever, and was hospitalized Thursday afternoon. He soon after tested positive for Ebola, and remains in isolation at Bellevue Hospital in Manhattan. His fiance and two friends have been placed in quarantine because of contact with him.

Spencer entered the U.S. through John F. Kennedy International Airport in Queens, where he and other passengers traveling from West Africa had their temperatures taken as part of Ebola-screening efforts adopted last week. Afterward, he had taken his own temperature twice daily, according to the Doctors Without Borders aid group that he had worked with in Guinea.

Also Friday, New York Mayor Bill de Blasio reiterated that "there is no cause for alarm" after Spencer's diagnosis.

"New Yorkers need to understand that the situation is being handled, and handled well," de Blasio said.

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Also Friday, a Dallas nurse who had tested positive for Ebola celebrated being declared "virus-free" by getting a warm hug from President Barack Obama in the White House.

Nina Pham's dramatic visit to the Oval Office came hours after she was discharged from the National Institutes of Health in Bethesda, Md.

Pham, the second of two Dallas nurses to recover from Ebola, earlier was greeted by applause from staff standing outside NIH.

Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, "Nina Pham is free of Ebola virus." He cited multiple tests that found no trace of the disease in recent days.

"She feels well . . . she looks extraordinarily well," Fauci said.

"I feel fortunate and blessed to be standing here today," Pham told reporters there. "Through this ordeal I put my trust in God, and my medical team . . . Of course, I am so terribly thankful for everybody involved in my care."

"I believe in the power of prayer, because I know so many people around the world have been praying for me," she said.

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"I join you in prayer now for the recovery of others, including my colleague, Amber Vinson, and Dr. Craig Spencer," Pham said before going to the White House to meet Obama.

Vinson, Pham's co-worker, was declared free of the virus earlier this week. But she remains hospitalized in Atlanta.

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Pham and Vinson contracted Ebola while treating an Ebola patient in Dallas named Thomas Eric Duncan, a Liberian national who died Oct. 8. More than 100 people who had contact with Duncan and the two nurses were tracked and monitored as a result.

No one else connected to the Dallas mini-outbreak has developed symptoms of the virus. The lack of infection among his family members and own fiance after being in close contact with him has been cited by health experts as evidence of how difficult it is to transmit Ebola.

But the rapid recovery of Vinson and Pham has surprised many, because the mortality rate for the current epidemic in West Africa has topped 50 percent. Nearly 5,000 people have died from the disease during this outbreak, the worst on record. A 2-year-old girl in Mali, who was the first confirmed case of Ebola in that country, died Friday.

There currently is no approved vaccine or approved treatment for Ebola. But authorities have fast-tracked the development of several vaccines, and also have employed some experimental drug treatments on Ebola patients hospitalized in the U.S.

Pham did not receive an experimental drug treatments. But like several other patients, Pham received plasma transfusion from Dr. Kent Brantly, an American physician who had recovered from Ebola.

The transfusions were done in the hope that Ebola antibodies in Brantly's blood might help others fight the virus.

It is not known if the transfusions have played any role in Pham's recovery, or any others

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But Pham said on Friday, "I would especially like to thank Dr. Kent Brantly for this selfless act of donating his plasma to me."

She also said, "Although i now longer have Ebola, I know it may be a while before I get my strength back."

Before she spoke, a clean-up crew was preparing to enter Spencer's apartment in upper Manhattan.

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Officials earlier Friday revealed Spencer was not as feverish as first reported when he went to the hospital Thursday.

Spencer originally was said to have a temperature of 103 degrees on Thursday morning, when he first notified an international medical aid group that he had worked with in West Africa that he was running a fever.

But New York City health commissioner Mary Travis Bassett on Friday said Spencer's temperature was actually 100.3 degrees on Thursday when he was admitted to Bellevue.

Bassett, appearing on the MSNBC show "Morning Joe," said Spencer's temperature had been incorrectly transcribed Thursday.

Dr. Scott Gottlieb, an American Enterprise Institute resident scholar, told CNBC, that "some people don't even present with a fever, they present with other symptoms before they develop a fever,"

Gottlieb added that the fact that Spencer tested positive for Ebola so quickly could indicate he had high levels of the virus in his body when he arrived at the hospital.

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Bassett and other officials on Thursday night revealed that Spencer had not quarantined himself when he returned from Guinea.

Instead, Spencer had contact with four other people, rode several city subway lines, went for a three mile jog, ate at a restaurant and visited the popular High Line elevated walkway in Manhattan's Chelsea section. On Wednesday night, Spencer and his fiance went to a Brooklyn bowling alley, on the subway.

An Uber car driver whose vehicle was hired by Spencer is not considered at risk, officials said.

They also said they do not believe Spencer was a threat to his neighbors, since he was not symptomatic until Thursday morning, and because Ebola is transmitted only through contact with an infected person's bodily fluids.

Bassett also said there was "close to nil" chance that Spencer presented a danger of infecting someone from having ridden the subway.

The transit system's operator, the MTA, in a statement said, "The New York City Subway system is safe to ride. The person diagnosed with Ebola in New York City rode the subway several times since returning from abroad, but the state and city health commissioners agree there was no risk to any other subway customers or any MTA employees."

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Ebola is spread only by contact with the bodily fluids of a contagious person, and the virus cannot live for more than a few hours on hard surfaces," the MTA said "There is no indication the patient was contagious when he rode the subway. There is no indication he emitted any bodily fluids on the subway. There were no reports of bodily fluids on any of the subway lines he rode."

A neighbor of Spencer's said, "I'm worried about him."

But the neighbor, Tanya Thomas, 47, said she wasn't afraid of catching Ebola.

"No. If I get it, I get it. If I don't, I don't," Thomas told reporters reporters outside Spencer's apartment building in the Hamilton Heights neighborhood.

"I'm more concerned for him than I am for myself. I mean, he's the one with Ebola."

Spencer is a fellow of international emergency medicine and on the staff of Columbia University-New York Presbyterian Hospital.

In a statement, New York Presbyterian said, "He has not been to work at our hospital and has not seen any patients at our hospital since his return from overseas."

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The hospital system called Spencer a "dedicated humanitarian ... who went to an area of medical crisis to help a desperately underserved population. He is a committed and responsible physician who always puts his patients first.

"Our thoughts are with him, and we wish him all the best at this time," the hospital statement said.

New York officials said "The Gutter," the Williamsburg bowling alley that Spencer went to Wednesday night on the A and L subway trains, has closed temporarily in "an abundance of caution."

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In a statement release Friday morning, The Gutter said, "We voluntarily decided to close The Gutter yesterday evening as a precautionary measure while we gathered more information. We are workingwith the NYC Health Department to have the bar cleaned and sanitized under their supervision and expect to be open sometime today after that is completed."

"Doctors advising the Health Department have told us that our staff and customers were at no risk."

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University of Pennsylvania Professor Jeremy Siegel said he thinks that the recent gains against the disease-including the recent declaration that Nigeria is now Ebola-free-have led to "much less anxiety" from the New York case compared to previous U.S. infections.

Tevi Troy, former deputy secretary of the U.S. Health and Human Services, told CNBC in a "Squawk Box" interview on Friday that New York City is the most likely city where Ebola would have appeared given how much traffic goes through New York.

"It sounds like New York is mostly prepared," he said, "The hospital that Dr. Spencer is at is one of the hospitals that's prepared for this in New York."

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He noted, however, that caring for Ebola patients is extremely labor-intensive.

"The patient in Dallas supposedly had about 40 health care workers, which is a lot for a single patient," he said. "And you wonder, if you have multiple cases, to what extent does this begin to strain the health-care system, not just in terms of the tracking and tracing but also the protective equipment and the number of health-care workers who are monitoring the person?"

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The World Health Organization said on Friday it was sending experts to help Mali fight Ebola, hours before the 2-year-old victim of the disease there died.

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Malian authorities are monitoring 43 people who were in contact with the girl, including 10 health workers, a WHO official told reporters.

-Reuters contributed to this report.