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, ABCnews reported.

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.

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.

Still, the lone U.S. Ebola patient is in critical condition, the Dallas hospital that has been treating him reported Saturday.

Duncan travelled from disease-ravaged Liberia to Dallas last month before he began showing symptoms of the disease.

Health officials said Saturday that they are monitoring about 50 people for signs of the deadly disease who may have had contact with Duncan, including nine who are believed to be at a higher risk. Thus far none have shown symptoms. Among those being monitored are people who rode in the ambulance that transported Duncan back to the hospital before his diagnosis, said Dr. Tom Frieden, the director of the Centers for Disease Control and Prevention.

The family who lived there was moved to a private home in a gated community, where they are being carefully monitored. The city had been having trouble finding a place that would take in Louise Troh, originally from Liberia, her 13-year-old son and two nephews, Voice of America reminds.