Dr. Craig Spencer, the Doctors Without Borders volunteer who was infected with Ebola while treating patients in Guinea, is free of the virus and will be released Tuesday, New York City health officials say.

"Dr. Spencer poses no public health risk and will be discharged from the hospital tomorrow, Tuesday, November 11th," the city health department said in a statement.

Spencer, 33, returned from Guinea on Oct. 17 and followed federal protocols by taking his temperature twice a day. He called Doctors Without Borders when he developed a temperature and Bellevue was able to send specially garbed ambulance workers to get him. The hospital had been preparing for Ebola patients and had a special unit set up to treat him.

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But some criticized Spencer for having traveled around the city before he got ill, and officials are still monitoring his contacts. New York's health department is monitoring 357 people, mostly travelers but also a few staff who have been caring for Spencer, as well as ambulance staff who brought him to the hospital.

Spencer received a plasma transfusion from Nancy Writebol, a medicial missionary who also survived Ebola infection.

Transfusions of blood from survivors have been used to help treat other Ebola patients, including NBC News camera operator Ashoka Mukpo and medical missionaries Dr. Rick Sacra and Dr. Kent Brantly. All the U.S. patients also got experimental drugs, although doctors all stress that it’s impossible to say what treatment helped any of them survive. They say supportive care such as fluid and salt replacement was also important.

The U.S. is now free of known Ebola cases. Only two people have been infected in the United States and the 21-day monitoring period is almost over for Spencer’s contacts; it will end Thursday. As predicted by the Centers for Disease Control and Prevention and many other experts, the people at greatest risk were health care workers in close, prolonged contact with a patient who was actively having symptoms.

The 21 days is based on the longest known incubation period for Ebola. Experts say most transmissions happen between 6 and 12 days.

They also point out that patients only transmit the virus after they’ve developed symptoms, usually vomiting or diarrhea but also — not likely but possibly — an initial fever.

-Maggie Fox