Story highlights A Liberian man who was quarantined in Lagos, Nigeria, dies of Ebola

Lagos has more than 20 million residents

The current outbreak of Ebola virus is the deadliest ever, health officials say

The outbreak has been centered in Guinea, Sierra Leone and Liberia

A Liberian man who was hospitalized in Nigeria's biggest city, Lagos, with the Ebola virus has died, Nigerian Health Minister Onyebuchi Chukwu said.

"The patient was subjected to thorough medical tests ... which confirmed he had the Ebola virus," Chukwu said Friday.

The case has raised fears that the virus could spread beyond the three countries at the center of what health officials say is the deadliest ever Ebola outbreak and into Africa's most populous nation, Nigeria. Lagos has more than 20 million residents.

As of July 20, some 1,093 people in Guinea, Sierra Leone and Liberia are thought to have been infected by Ebola since its symptoms were first observed four months ago, according to the World Health Organization . Testing confirmed the Ebola virus in 786 of those cases, of whom 442 died.

Of the 1,093 confirmed, probable and suspected cases, 660 people have died.

The man hospitalized in Lagos was a 40-year-old Liberian working for a West African organization in Monrovia, Liberia, according to the Lagos State Ministry of Health.

He arrived at Lagos airport on Sunday and was isolated in a local hospital after showing symptoms associated with the virus. He told officials that he had no direct contact with anyone with the virus nor attended the burial of anyone who died of Ebola.

The Lagos State Ministry of Health had said Thursday that "the patient's condition is stable and is in recovery" and that the results of testing for Ebola infection were still pending in his case. Infection control measures were in place in the hospital, officials said.

Doctor infected

Confirmation of the death in Lagos followed news that a doctor who has played a key role in fighting the Ebola outbreak in Sierra Leone is infected with the disease , according to that country's Ministry of Health.

Dr. Sheik Humarr Khan is being treated by the French aid group Medecins Sans Frontieres -- also known as Doctors Without Borders -- in Kailahun, Sierra Leone, agency spokesman Tim Shenk said.

Before falling ill, Khan had been overseeing Ebola treatment and isolation units at Kenema Government Hospital, about 185 miles east of the capital Freetown.

Ebola typically kills 90% of those infected, but the death rate in this outbreak has dropped to roughly 60% thanks to early treatment.

Spread by bodily fluids

Officials believe that the Ebola outbreak has taken such a strong hold in West Africa due to the proximity of the jungle -- where the virus originated -- to Conakry, Guinea, which has a population of 2 million.

Because symptoms don't immediately appear, the virus can easily spread as people travel around the region. Once the virus takes hold, many die in an average of 10 days as the blood fails to clot and hemorrhaging occurs.

The disease isn't contagious until symptoms appear. Symptoms include fever, headache and fatigue. At that point, the Ebola virus is spread via bodily fluids.

Health workers are at especially high risk, since they are in close contact with infected people and their bodily fluids. Adding to the danger, in the initial stages of infection doctors may mistake an Ebola infection for another, milder illness.