 -- Dr. Rick Sacra, a family physician from Massachusetts who survived Ebola, was readmitted to a hospital today with what doctors said appears to be an upper respiratory infection.

UMass Memorial Medical Center physicians said they believe it is likely Sacra is not suffering a relapse of Ebola, but in the interest of safety they are isolating him until they have confirmation.

Sacra, 51, underwent treatment at the Nebraska Medical Center after contracting the deadly virus while treating patients in Liberia. He was released on Sept. 26.

"Even though the likelihood of Dr. Sacra having a relapse of Ebola is extremely low, doctors will run tests to be 100 percent sure,” Dr. Phil Smith, who treated Sacra in Nebraska, told ABC News affiliate WCVB in Boston. "The symptoms he has are indicative of a respiratory illness and are not those of someone suffering from Ebola."

Doctors at UMass Medical Center also believe he is suffering from an upper respiratory infection and not a relapse of Ebola, but they have placed him in isolation, following guidance from the Centers for Disease Control and Prevention, until they get test results back from the CDC, hospital officials said.

"Dr. Sacra is in stable condition and being monitored carefully. We're waiting for final test results from the CDC which we expect to receive late Monday," said Dr. Robert Finberg, MD, professor and chair of medicine, UMass Memorial Medical Center, an infectious disease expert leading Dr. Sacra's team of doctors.

"We are isolating Dr. Sacra to be cautious pending final confirmation of his illness," Finberg said. "We think it is highly unlikely that he has Ebola. We suspect he has an upper respiratory tract infection, with symptoms of cough and conjunctivitis."

Sacra was the third of four American health workers who have been brought to the U.S. after being infected with Ebola, since the outbreak started in West Africa in March.

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