LAGOS, Nigeria — Nigerian authorities say they have confirmed a second case of Ebola in Africa’s most populous country, an alarming development after a man who flew by plane to the country died of Ebola.

Nigerian Health Minister Onyebuchi Chukwu said Monday that the second person with Ebola is a doctor who had helped treat Patrick Sawyer, the Liberian-American man who died of Ebola in late July.

Sawyer, who was traveling to Nigeria on business, became ill while aboard a flight and Nigerian authorities immediately took him into isolation. They did not quarantine his fellow passengers, and have insisted that the risk of additional cases was minimal.

Nigeria is the fourth country to report Ebola cases and at least 728 other people have died in Guinea, Sierra Leone and Liberia.

Two Americans who have been diagnosed with Ebola will be treated in the U.S. The first patient arrived in Atlanta over the weekend. The second patient, who is a missionary, is scheduled to leave Liberia for the U.S. on Tuesday.

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