The spread of the deadly Ebola virus in West Africa prompted the U.S. Centers for Disease Control and Prevention on Thursday to raise its travel health alert to Level 3, its highest level, warning people to avoid nonessential travel to the three countries experiencing the current outbreak.

CDC Director Tom Frieden told reporters in a news briefing that the agency will also send 50 additional personnel over the next 30 days to Guinea, Liberia and Sierra Leone, to help the 12 staff already on the ground in those countries.

Since the first report of the deadly virus surfaced in March, there have been 1201 cases reported and 672 deaths, according to the World Health Organization.

“The bottom line is that ebola is worsening in West Africa,” he said. Both the CDC and the World Health Organization are “surging our response to begin to turn the tide,” he said, referring to deployment of additional personnel. “It’s not going to be quick and it’s not going to be easy. But we know what to do.”

The agency’s previous Level 2 travel health alert advised U.S.-based health-care workers to be aware of the symptoms of Ebola and the travel histories of their patients. Two American health-care workers, volunteers with the Christian aid group Samaritan’s Purse, have contracted the virus and are in “stable but grave” condition, according to a statement released by the charity.

The group has since evacuated some workers from Liberia, but medical staff have been left behind to treat patients. Sierra Leone’s top Ebola doctor fell victim to the virus and died.

The worst-ever Ebola outbreak has prompted the Peace Corps to evacuate volunteers from the three West African countries, the organization has announced. Sierra Leone has declared a state of emergency and is mobilizing police and the military to quarantine the epicenters of the disease.

The Ebola virus is spread via direct contact with bodily fluids, and people are considered contagious when they start showing symptoms. It can take as long as 21 days before a person infected with the virus starts showing symptoms, according to the World Health Organization.

While the CDC has said that the odds that Ebola will reach the United States are slim, it has still advised American health-care workers to be cautious. “The likelihood of this outbreak spreading outside of West Africa is low,” Stephan Monroe, deputy director of CDC’s National Center for Emerging and Zoonotic Infectious Diseases, said on a conference call with reporters Monday. But, he added, “the CDC has to be prepared for the remote possibility.”

The Peace Corps will temporarily remove its 102 volunteers in Guinea, 108 in Liberia and 130 in Sierra Leone. The group had previously released a statement saying that it was monitoring the outbreak in consultation with the CDC and the State Department.