Lagos recorded its second case of Ebola on Monday, in a doctor who treated U.S. victim Patrick Sawyer, Nigeria's health minister said.
Sawyer died in Lagos last month after arriving there by plane from Liberia.
Ebola has killed 826 people in West Africa since the outbreak began in February, mostly in Guinea, Liberia and Sierra Leone.
"As of today, one of the doctors that treated the late Mr Sawyer has tested positive for the Ebola virus," Onyebuchi Chukwu told a news conference. Of 70 people who were under surveillance, eight had been "quarantined at an isolation ward provided by the Lagos state government," he added.
Ebola is one of the world's deadliest diseases, with a mortality rate of up to 90 percent of cases. The disease starts with headaches and fever, and final-stage symptoms include external and internal bleeding, vomiting and diarrhea. There is no effective treatment and no vaccine to protect against it.
The latest outbreak began in the forests of remote eastern Guinea in February. Its arrival of one of the world's deadliest diseases in Nigeria, Africa's biggest economy and its most populous country, with 170 million people, has caused panic.
Lagos is Africa's biggest city, with 21 million people, and one of the world's most crowded, with poor healthcare infrastructure.