WASHINGTON, D.C. — The Trump administration is barring foreign travelers from entering the United States if they have been in the Democratic Republic of the Congo, Uganda or South Sudan within the past 21 days, according to a Centers for Disease Control and Prevention order issued the previous day. The 30-day restriction is the first travel ban ever imposed by the U.S. in response to an Ebola outbreak.

The order follows confirmation that an American doctor has been infected with the virus. The current outbreak has become the fourth largest on record, with cases reported among health care workers and infections documented in cities.

Speaking at the World Health Assembly in Geneva, World Health Organization Director-General Tedros Adhanom Ghebreyesus said there were more than 500 suspected Ebola cases and 130 suspected deaths. He linked the trajectory of the outbreak to instability in the region, where conflict has escalated over the past two months. "Over 100,000 people have been newly displaced, and in Ebola outbreaks, you know what displacement means," Tedros said.

The WHO and expert scientists are debating whether an existing vaccine developed for a different species of Ebola could help contain the outbreak. Decisions about deploying the vaccine will be left to the affected countries. Virologist Darryl Falzarano worked on an animal study that showed the existing vaccine had some cross-protection potential against the strain circulating in the current outbreak.

The WHO has been operating with a reduced budget, in part because of the United States' withdrawal from the agency under the Trump administration. The CDC order applies to all foreign nationals who have been present in the DRC, Uganda or South Sudan at any point in the 21 days preceding their planned entry into the U.S., and remains in effect for 30 days from the date of issuance.