WASHINGTON: The Centers for Disease Control and Prevention said Monday that the risk of Ebola spreading to the United States was low, but it nevertheless issued a Level 2 health alert, advising US health workers to be vigilant.

The largest outbreak of the Ebola virus on record has spread across the West African nations of Guinea, Sierra Leone and Liberia since first detected in March, with more than 670 deaths, according to the CDC.

Two Americans working in Liberia were diagnosed with the virus last week. That raised fears that the virus could be spread to the United States by those traveling from the region.

But the CDC played down the possibility. “The likelihood of this outbreak spreading outside of West Africa is low,” said Stephan Monroe, deputy director of the agency’s National Center for Emerging and Zoonotic Infectious Diseases. Still, he said in a teleconference with reporters, “the CDC has to be prepared for the remote possibility.”

The alert advised US doctors to check patients’ travel histories to identify those who might have recently traveled to West Africa. The CDC also urged physicians to learn the symptoms of Ebola, which include fever and diarrhea, and the immediate treatment necessary if they suspect it. The agency warned travelers going to West Africa to avoid contact with bodily fluids or blood of anyone in the affected areas.

Kent Brantly, an American physician who was infected, is undergoing treatment at the isolation ward at ELWA Hospital in the Liberian capital of Monrovia. As director of the Ebola Consolidated Case Management Center run by Samaritan’s Purse, a Christian charity, in Monrovia, Brantly worked closely with Ebola patients.

His wife and two children had been living with him in Liberia but returned recently to the United States, and they are being watched closely for symptoms as a precaution.