Guinea's government has for the first time confirmed cases of the deadly Ebola virus in the capital Conakry.
Until now, the 66 confirmed deaths have only been in rural areas, although there have been suspected cases, which have since proved negative, in the capital.
There have also been suspected cases in neighbouring West African states Liberia and Sierra Leone.
Ebola is spread by close contact and kills between 25% and 90% of victims.
Earlier this week, the health ministry banned the sale and consumption of bats, in a bid to prevent the spread of the virus. Fruit bats, which are a delicacy in the worst affected south-eastern region, are thought to be carriers of the disease.
Health Minister Remy Lamah said the virus appeared to have been transmitted by an man who showed symptoms of haemorrhagic fever after visiting Dinguiraye in central Guinea, far from the identified outbreaks of Ebola in the remote south-east.
Four of the man's brothers, who attended his funeral in the central town of Dabola, started to show the same symptoms and were tested for Ebola on their return to Conakry.
The four have been placed in an isolation ward and the dead man's family have also been quarantined, the minister said.
The spread of the disease to Conakry, a city of some two million people, marks an escalation in the Ebola outbreak in Guinea - one of the poorest nations on earth, despite rich deposits of bauxite and iron ore.
Discovered in 1976 after an outbreak in the Democratic Republic of Congo, then Zaire, Ebola causes a severe haemorrhagic fever where victims suffer vomiting, diarrhoea and both internal and external bleeding.
Scientists have yet to develop an effective drug or vaccine to fight it.
Part of the problem is that the deadly virus is rare and its victims are often poor people living in rural areas of Africa without well-functioning health systems. But there is also little incentive for major pharmaceutical companies to invest in medical solutions when there is little chance of a return, analysts say.
However, many health officials believe the virus could be better controlled with good basic hygiene and the eradication of dangerous bush meat consumption. The US government also funds some research, partly out of concern the virus could be used for bioterrorism.
"Ebola virus is one of the deadliest killers known," said Ben Neuman, a virologist at Britain's University of Reading.
"If this virus spread between people more easily, it would probably be more deadly than the black plague. Fortunately, up to this point, it has not," he added.
Outbreaks of Ebola occur primarily in remote villages in Central and West Africa, near tropical rainforests, the World Health Organization says.