Ebola, one of the world’s most fatal diseases, has surfaced in Africa’s most populous country.

Nigerian health officials have announced 10 confirmed cases and two deaths in the country from the Ebola outbreak that is sweeping West Africa, including a nurse and a man from Liberia whom the nurse had been caring for.

The man, Patrick Sawyer, a naturalized American citizen, had flown to Nigeria in late July and died soon after. He had infected at least eight other people, including the nurse, who died on Tuesday, officials said.

By Friday, President Goodluck Jonathan had declared a state of emergency, officially adding Nigeria, home to more than 160 million people, to the list of nations struggling to control one of the largest public-health emergencies in recent history. More than 900 people have died.

The story of Mr. Sawyer, who according to Nigerian newspaper accounts was aware that he was sick when he left Liberia, demonstrates just how difficult containing the disease will be in the modern age of rapid travel and growing urbanization. Nigeria has Africa’s largest economy and is deeply connected to the outside world, a fact that could magnify the consequences if the outbreak is not contained.