Concerns over the recent outbreak of the deadly Ebola virus in West Africa prompted the U.S. Centers for Disease Control and Prevention to issue a travel advisory to the area. The CDC on Thursday raised 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 1,201 cases reported and 672 deaths, according to the World Health Organization.

“The bottom line is that ebola is worsening in West Africa,” he said. He described the virus as “dreadful” and “merciless.” He added: “The current outbreak is bad. It’s the biggest and most complex and first time it’s present in this region of the world.”