A healthcare worker who treated Ebola patients in West Africa and was placed in isolation in New Jersey after developing a fever has tested negative for the virus but will remain in quarantine for 21 days, health officials said Saturday. The worker, a staffer for Medecins Sans Frontieres, or Doctors Without Borders, was placed into quarantine Friday upon arrival at New Jersey’s Newark International Airport, a day after a New York City doctor who also worked with the group overseas tested positive for the deadly disease. "Physicians at University Hospital continue to monitor the patient and consult with the Department of Health and the CDC on patient evaluation and any potential need for additional testing," the New Jersey Department of Health said in a statement.

Medecins Sans Frontieres confirmed the worker is one of its staffers. The worker initially had no symptoms when placed in quarantine, but developed a fever Friday night and was placed in isolation at University Hospital in Newark, the New Jersey Department of Health said in a statement. The potential new case comes after Dr. Craig Spencer became ill Thursday in New York City and later tested positive for the deadly disease, which has killed more than 4,800 people worldwide, mostly in the West African countries of Liberia, Guinea, and Sierra Leone.

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"This evening, that health care worker developed a fever & is in isolation &being evaluated at University Hospital Newark." @NJDeptofhealth — Governor Christie (@GovChristie) October 25, 2014

Ebola is an extremely hard disease to contract. It is only transmitted through bodily fluids – not simply by being near an infected person. — Bill de Blasio (@BilldeBlasio) October 24, 2014

— Phil Helsel