A second US nurse diagnosed with Ebola while caring for a patient from Liberia was released from Emory University Hospital in Atlanta Tuesday after being declared free of the virus.

"Today, I am pleased to announce that Amber Vinson is being discharged from Emory University Hospital," Bruce Ribner, director of Emory's Infectious Disease Unit, told reporters at a press conference.

"After a rigorous course of treatment and thorough testing, we have determined that Ms. Vinson has recovered from her infection with Ebola virus, and she can return to her family, to the community and to her life without any concerns," Ribner said.

Vinson, 29, who arrived at the hospital on Oct. 15, read a brief statement at the conference but didn't answer questions.

"I am so grateful to be well," Vinson said. "As a nurse, and now someone who has experienced what it is like to be cared for through a life-threatening illness, I am so appreciative and grateful."

Vinson thanked all those who contributed to her care, especially Kent Brantly and Nancy Writebol, the first two to be cured of the disease at Emory, for their donations of blood plasma to herself and other patients with Ebola.

She also thanked her grandparents, her aunt and her uncle who were present at the conference.

Vinson and her fellow nurse Nina Pham contracted the virus while treating patient Thomas Eric Duncan, a Liberian who died on Oct. 8, at Texas Health Presbyterian Hospital.

Pham, 26, was later transferred to a hospital at the US National Institutes of Health for treatment. She was released from the hospital on Friday after being declared free of the virus.

When asked what contributed to the speedy recovery of both nurses, Ribner said: "The honest answer is that we're not exactly sure."

But he said their younger ages may be one factor. "They are two of the youngest patients who have been treated in developed countries," he said. "We know from a lot of data coming out of Africa that younger patients do much better than patients who are older."

Ribner also assumed that their protective equipment they wore while caring for Duncan may also have played a role.

"It's quite likely the amount of virus she was exposed to was substantially less than what we see in patients who got infected in less developed countries. We also know the higher the viral load that you got infected with, the more severe your disease is likely to be."

Vinson is the fourth patient to be successfully treated for Ebola at Emory, and the seventh treated in the United States to fully recover. Only the Liberian, Duncan, died of the virus.

A doctor, who recently returned to New York City after treating Ebola patients in West Africa and tested positive for the virus on Thursday, is now the sole person in the country to remain in the hospital for treatment.