Nigeria has confirmed its first Ebola case in more than a decade, after a woman who arrived from Uganda tested positive for the virus in the capital, Abuja. The health ministry said the result was confirmed by laboratory tests on Tuesday, making Nigeria the first West African country drawn into the outbreak that began in the Democratic Republic of the Congo in May.

The patient developed symptoms on 7 October, four days after arriving in the country, according to the World Health Organization. She sought care at a private hospital in Abuja on 11 October and was kept there as her condition worsened. Ebola was confirmed on 20 October, and she is now in isolation and receiving treatment.

Health Minister Muhammad Ali Pate said contact tracing began immediately, with 44 close contacts identified so far. Most are healthcare workers. "We are doing everything possible to contain this case by tracing all the contacts the individual may have had," Pate said. He told the national broadcaster NTA that the woman had shown no symptoms when she landed and that she had not confirmed travelling to Congo herself. "We presume that it was likely picked in Uganda," he said.

Uganda's government pushed back on that assumption. The director general of health services, Charles Olaro, said preliminary information showed the woman had travelled to eastern Congo for business before passing through Uganda in early October, and that her symptoms began only after she reached Nigeria. "Travel through Uganda does not establish that infection occurred here. The location of exposure remains under investigation," Uganda's health ministry said. It added that the country has no confirmed active Ebola cases and urged the public to stay calm and keep washing hands.

The two accounts leave an important question open. Uganda says Nigeria has not yet supplied a detailed travel itinerary and clinical history. Nigeria has not disclosed the woman's nationality. The BBC, citing health sources, reported that she is not Nigerian and is the wife of a diplomat, a detail the authorities have not confirmed publicly. Her route and the point at which she was exposed are central to deciding where else officials must look for infections.

The outbreak behind the case is caused by the Bundibugyo species of Ebola, which is rare and has no approved vaccine or treatment, although trials are under way. The Africa Centres for Disease Control and Prevention says the fatality rate is nearly 50%. More than 9,500 cases and at least 4,600 deaths have been recorded in Congo since May, according to figures from the European Centre for Disease Prevention and Control cited by the BBC. The Guardian reported that the WHO expects the outbreak to surpass the 11,000 deaths recorded in West Africa between 2014 and 2016, the deadliest Ebola epidemic on record.

The virus has spread steadily across the continent. Uganda reported 20 cases in May and June, most of them imported from Congo, and was declared free of the disease in August after its last patient was discharged. Kenya confirmed its first case this month in a traveller who had been in Congo, and has since recorded nine infections linked to that first patient. The spread to Kenya raised questions about how well border screening is working. On Monday, the United States advised its citizens to reconsider travel to Kenya because of the health risk.

Nigeria has prepared for this moment since June, when it set up an Ebola taskforce after the outbreak was declared in Congo. The Nigeria Centre for Disease Control stepped up surveillance when Kenya reported its first case, focusing on early detection, isolation, laboratory diagnosis, infection control and contact tracing. Pate pointed to the country's record in 2014, when it stopped an imported outbreak that killed eight people during the West African epidemic. "We have the systems in place," he said.

What happens next depends largely on the 44 contacts. Ebola's incubation period runs from two to 21 days, so anyone exposed may fall ill weeks from now. Public health teams are monitoring the contacts, and officials are using the woman's travel documents to trace passengers who shared her flights. Those passengers and the hospital staff who treated her before the diagnosis are the groups most likely to need follow-up testing.

For the region, the case is a test of whether the outbreak can be kept out of Africa's most populous country, with its dense cities and busy airports. Africa CDC Director General Jean Kaseya said the outbreak is now the most geographically extensive in history and called it "an extremely serious threat" that needs political leadership and funding. What remains unknown is whether this is a single imported case or the start of local transmission, and how many of the contacts will test positive in the coming three weeks.