Dr. Craig Spencer, New York's first resident to have been diagnosed of Ebola infection, is now in serious but stable condition after receiving blood plasma transfusion, a health official said Sunday.

Dr. Ram Raju, director of the corporation operating Manhattan's Bellevue Hospital, said in a news briefing that Spencer's immune system responded well to plasma therapy. He was announced to have progressed to a more serious phase of the infection the day before.

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Citing patient privacy, Dr. Raju declined to offer more details on Spencer's condition. However, he indicated that the doctor is undergoing the expected course of the disease as of early Sunday.

While officials did not elaborate on the source of the plasma, ABC News reported that Nancy Writebol, an American Ebola survivor who was repatriated from Liberia in August, donated the blood.

She contracted the disease during her missionary work in Liberia, where 4,665 people have already been diagnosed of the disease as of latest World Health Organization tally.

Writebol was cleared of the virus on Aug. 19, following treatment at Emory University Hospital in Atlanta.

Spencer is a returning volunteer for Doctors Without Borders from Guinea, one of the three Ebola-stricken West African countries.

He was diagnosed last Thursday at Bellevue after emergency workers rushed to his apartment on report that he registered 100.3 degrees Fahrenheit-a possible indication of infection.

According to Doctors Without Borders protocol, volunteers like Spencer are advised to take their daily temperatures within 21 days of their return from missionary outposts.

His fiancée, Morgan Dixon, 30, was released from Bellevue on Saturday night after doctors cleared her of any Ebola symptom. Dixon remains quarantined in the doctor's apartment and was advised not to leave home or receive visitors until after the maximum Ebola virus incubation is over. 

In a statement released on Sunday, New York City Mayor Bill de Blasio said he had talked with Dr. Spencer who he said is a "noble" person concerned for other people's welfare, ABC News quoted him as saying. 

TagsNew York City ebola, Craig Spencer, plasma treatment, what is ebola