ABUJA, Nigeria (AP) ― Nigerian health authorities raced to stop the spread of Ebola on Saturday after a man sick with one of the world’s deadliest diseases brought it by plane to Lagos, Africa’s largest city with 21 million people.

The fact that the traveler from Liberia could board an international flight also raised new fears that other passengers could take the disease beyond Africa due to weak inspection of passengers and the fact Ebola’s symptoms are similar to other diseases.

Officials in the country of Togo, where the sick man’s flight had a stopover, also went on high alert after learning that Ebola could possibly have spread to a fifth country.

Screening people as they enter the country may help slow the spread of the disease, but it is no guarantee Ebola won’t travel by airplane, according to Dr. Lance Plyler, who heads Ebola medical efforts in Liberia for aid organization Samaritan’s Purse.

“Unfortunately the initial signs of Ebola imitate other diseases, like malaria or typhoid,’’ he said.

The aid organization on Saturday said a U.S. doctor working with Ebola patients in Liberia had tested positive for the deadly virus. A Samaritan’s Purse news release said Dr. Kent Brantly was being treated at a hospital in Monrovia, the capital.

Ebola already had caused some 672 deaths across a wide swath of West Africa before the Nigeria case was announced. It is the deadliest outbreak on record for Ebola, and now it threatens Nigeria, Africa’s most populous nation. An outbreak in Lagos, Africa’s megacity where many live in cramped conditions, could be a major diisaster.

“Lagos is completely different from other cities because we’re talking about millions of people,’’ said Plan International’s Disaster Response and Preparedness Head, Dr. Unni Krishnan.

Nigerian newspapers describe the effort as a “scramble’’ to contain the threat after the Liberian arrived in Lagos and then died Friday.

International airports in Nigeria are screening passengers arriving from foreign countries for symptoms of Ebola, according to Yakubu Dati, the spokesman for Federal Aviation Authority of Nigeria.

Health officials are also working with ports and land borders, he said. “They are giving out information in terms of enlightenment, what to do, what to look out for.’’

And Nigerian airports are setting up holding rooms to ready in case another potential Ebola victim lands in Nigeria.