US BARS GREEN CARD HOLDERS FROM EBOLA-AFFECTED COUNTRIES AND ROUTES ALL ARRIVALS THROUGH FIVE DESIGNATED AIRPORTS AS FIRST INFECTED AMERICAN DESCRIBED AS "CAUTIOUSLY OPTIMISTIC"

Following the WHO's declaration of a Public Health Emergency of International Concern on May 17, the United States announced enhanced entry restrictions on May 18 aimed at preventing Ebola from entering the country.

Under the new measures, all travelers who have been in the Democratic Republic of Congo, Uganda, or South Sudan within the past 21 days must fly into one of five designated airports for mandatory health screening. Lawful permanent residents — green card holders — who have been in any of the three countries within that window are currently barred from re-entering the United States while the CDC completes its public health risk assessment. The restrictions are in effect for 30 days.

Travelers who show no symptoms after screening are permitted to continue to their final destination, but must monitor for symptoms for 21 days following their departure from an affected region.

The first American to be infected with the Ebola virus has been identified. The individual was described as critically ill but "cautiously optimistic," according to ABC News. The current outbreak involves the Bundibugyo strain, for which no approved vaccine or treatment exists.

Sources: NPR, CBS News, CDC, ABC News