An outbreak of the Ebola virus - which has already killed 59 people in Guinea - has reached the capital Conakry, the UN's children agency has warned.
Unicef said the haemorrhagic fever had spread quickly from southern Guinea, hundreds of kilometres away.
Scores of cases have been recorded since the outbreak began last month. There is no known cure or vaccine.
It is spread by close personal contact with people who are infected and kills between 25% and 90% of victims.
Symptoms include internal and external bleeding, diarrhoea and vomiting.
"At least 59 out of 80 who contracted Ebola across the West African country have died so far," a Unicef statement quoted by the AFP news agency.
"Over the past few days, the deadly haemorrhagic fever has quickly spread from the communities of Macenta, Gueckedou, and Kissidougou to the capital Conakry."
Conakry is a sprawling port city, where up to two million people currently live.
The Unicef statement also said that at least three victims of the virus were children.
Outbreaks of Ebola occur primarily in remote villages in Central and West Africa, near tropical rainforests, the World Health Organization says.
Analysts suggest it has never been recorded in Guinea before.
Recent years have seen outbreaks in Uganda and Democratic Republic of Congo.
"We got the first results from Lyon yesterday which informed us of the presence of the Ebola virus as the cause of this outbreak," Guinean health ministry official Sakoba Keita told AFP on Saturday.
"We are overwhelmed in the field, we are fighting against this epidemic with all the means we have at our disposal with the help of our partners but it is difficult."
Medical aid charity Medecins sans Frontieres said on Saturday it would strengthen its team in Guinea and fly some 33 tonnes of drugs and isolation equipment in from Belgium and France.
Dr Armand Sprecher, an emergency physician and epidemiologist working with MSF in Guinea, told the BBC that doctors had to identify all patients with the disease and monitor anyone they had been in contact with during their illness.
The latest outbreak could be brought under control if people acted quickly, he said.
"Based on our history with these sorts of outbreaks it will happen. Ideally, sooner rather than later," said Dr Sprecher.
"The more quickly we can contain this the fewer cases we'll have, then the smaller the scale of the epidemic. That's the idea of going in as strong as we can early on."