Nigerian health authorities raced to stop the spread of Ebola after a man sick with one of the world's deadliest diseases brought it by plane to Lagos.

That the traveller from Liberia could board an international flight also raised new fears that other passengers could take the disease beyond Africa due to weak inspection of passengers and the fact Ebola's symptoms are similar to other diseases.

Officials in Togo, where the sick man's flight had a stopover, also went on high alert after learning that Ebola could possibly have spread to a fifth country.

Screening people as they enter the country may help slow the spread of the disease, but it is no guarantee Ebola will not travel by aeroplane, according to Dr Lance Plyler, who heads Ebola medical efforts in Liberia for aid organisation Samaritan's Purse.

"Unfortunately the initial signs of Ebola imitate other diseases, like malaria or typhoid," he said.

The aid organisation said yesterday that a US doctor working with Ebola patients in Liberia had tested positive for the deadly virus.

It said Dr Kent Brantly was being treated at a hospital in Monrovia, the capital, where he had been serving as medical director for the organisation's case management centre.

Photos of Dr Brantly working in Liberia show him in white coveralls made of a synthetic material that he wore for hours a day while treating Ebola patients.

Dr Brantly was quoted on the organisation's website earlier this year about efforts to maintain an isolation ward for patients.

"The hospital is taking great effort to be prepared," he said.

"In past Ebola outbreaks, many of the casualties have been healthcare workers who contracted the disease through their work caring for infected individuals."

Samaritan's Purse spokeswoman Melissa Strickland said Dr Brantly's wife and children had been living with him in Africa but are currently in the US.

Ebola already had caused some 672 deaths across a wide swath of West Africa before the Nigeria case was announced.

It is the deadliest outbreak on record for Ebola, and now it threatens Nigeria, Africa's most populous nation.

An outbreak in Lagos, Africa's largest city with 21 million people, and where many live in cramped conditions, could be a major disaster.

"Lagos is completely different from other cities because we're talking about millions of people," said Dr Unni Krishnan of Plan International.

Nigerian newspapers describe the effort as a "scramble" to contain the threat after the Liberian arrived in Lagos and then died on Friday.

International airports in Nigeria are screening passengers arriving from foreign countries for symptoms of Ebola, according to Yakubu Dati, the spokesman for Federal Aviation Authority of Nigeria.

Health officials are also working with ports and land borders, he said.

"They are giving out information in terms of enlightenment, what to do, what to look out for."

And Nigerian airports are setting up holding rooms to ready in case another potential Ebola victim lands in Nigeria.

Airports in Guinea, Liberia and Sierra Leone, the three other West African countries affected by the current Ebola outbreak, have implemented some preventive measures, according to officials in those countries. But none of the safeguards is foolproof, say health experts.

Doctors say health screens could be effective, but Ebola has a variable incubation period of between two and 21 days and cannot be diagnosed on the spot.

Patrick Sawyer, a consultant for the Liberian ministry of finance arrived in Nigeria on Tuesday and was immediately detained by health authorities suspecting he might have Ebola, Dr Plyler said.

On his way to Lagos, Mr Sawyer's plane also stopped in Lome, Togo, according to the World Health Organisation.

Authorities announced on Friday that blood tests from the Lagos University Teaching Hospital confirmed he died of Ebola earlier that day.

Mr Sawyer reportedly did not show Ebola symptoms when he boarded the plane, Dr Plyler said, but by the time he arrived in Nigeria he was vomiting and had diarrhoea.

There has not been another recently recorded case of Ebola spreading through air travel, he added.

Nearly 50 other passengers on the flight are being monitored for signs of Ebola but are not being kept in isolation, said an employee at Nigeria's ministry of health.

Dr Sawyer's sister also died of Ebola in Liberia, according to Liberian officials, but he claimed to have had no contact with her.

Ebola is highly contagious and kills more than 70% of people infected.

There is no "magic bullet" cure for Ebola, but early detection and treatment of fluids and nutrition can be effective, said Dr Plyler.

Quickly isolating patients who show symptoms is also crucial in slowing the spread of the disease.

West African hospital systems have weak and "often paralysed" health care systems, he added, and are not usually equipped to handle Ebola outbreaks.

International aid organisations like his and Doctors Without Borders have stepped in, but they also lack enough funding and manpower.

"We need more humanitarian workers," he said. "We need resources."

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