The number of Ebola cases worldwide has passed 10,000, according to the World Health Organization (WHO), as the outbreak continues to spread.
Authorities in Mali were scrambling to contain fears yesterday (Saturday) after the death of a two-year-girl who had travelled 700 miles on the country's bus network before receiving treatment.
President Barack Obama tried to reassure a panicking public in the US after two states imposed compulsory quarantine for doctors and nurses arriving home after treating patients overseas.
The health organisation says the number of cases is growing exponentially in Guinea, Liberia and Sierra Leone.
The outbreak is the worst in history. Yesterday's statistics put the total at 10,141. Of those, 4,922 people had died.
The United Nations' health arm has said repeatedly that even those figures are likely to underestimate the true scale of the outbreak as many people have been unable or too frightened to seek medical care.
Four WHO experts are on their way to Mali, the sixth West African country to record an Ebola case, to reinforce its team there.
Markatche Daou, a spokesman for the Malian ministry of health, said the girl had arrived from Guinea, where she had visited Kissidougou, the town where the outbreak was first identified last December.
"Everyone who had contact with the girl is under medical surveillance," he said.
Health organisation experts warned that many people in Mali had potentially been exposed to the virus because the toddler travelled across the country while she displayed symptoms. More than 40 people with whom she was in contact, including 10 health care workers, are being monitored for symptoms such as fever.
So far, only 27 cases have occurred outside the most badly hit three countries. But the WHO has identified 15 other African nations at high risk of importing the disease.
However, hopes have been raised for trials of vaccines, which could begin in West Africa in December, earlier than expected, and hundreds of thousands of doses should be available for use by the middle of next year.
Meanwhile, authorities in New York have begun disinfecting bars and restaurants visited by Craig Spencer, a 33-year-old doctor, who tested positive for the disease on Thursday night.
The case sparked anxiety among New Yorkers, who complained that Dr Spencer should have been quarantined on returning from Guinea, where he had been treating Ebola patients with the medical charity Medecins Sans Frontieres.
Biohazard teams have cleaned Dr Spencer's apartment and a bowling alley he visited shortly before being taken ill.
Friends said he was well enough to practise yoga in hospital.
Dr Mary Travis Bassett, the city health commissioner, defended Dr Spencer, pointing out that he had quarantined himself as soon as he displayed symptoms.
"There's this young guy who went over there, really doing the right thing, the courageous thing, and he handled himself really well," she told The New York Times. "I don't want anyone portraying him as reckless."
At the same time, health officials in New Jersey said a nurse who developed a fever on Friday after returning from West Africa had tested negative for Ebola.
She was the first person to be quarantined on arrival at a US airport, following the introduction of tough new rules in New York and New Jersey.
They were imposed amid criticism that Washington was not doing enough to prevent the disease being carried into the US and calls for a complete ban on passengers arriving from West Africa.
Andrew Cuomo, governor of New York, said: "A voluntary Ebola quarantine is not enough. This is too serious a public health situation to leave to the honour system of compliance.
"I think increasing the screening procedures is necessary and reduces the risk to New Yorkers and people in New Jersey."
Health workers who have had contact with Ebola patients now face 21 days in isolation when they arrive back in the two states.
President Obama used his weekly Saturday address to reassure Americans. He said the disease was difficult to catch and that all seven Americans who had contracted Ebola had been treated successfully.