Senegal has closed its border with Guinea. About half a dozen suspected cases and two confirmed cases have been identified in neighboring Liberia, officials said. The center of the epidemic remains in Guinea’s remote forest region, around the towns of Macenta and Guéckédou, where isolation wards have been set up.

The Ebola virus is rare but deadly. Its point of origin is often the consumption of bush meat, including meat from apes or possibly bats, and it has a fatality rate of up to 90 percent. Human transmission occurs through contact with bodily fluids. Already, the Guinea outbreak is more serious than the most recent previous one, in Uganda in 2012, when fewer than 50 died. In that outbreak, cases were also recorded in the capital, Kampala. But in some previous outbreaks in Central and Eastern Africa, as many as 400 cases were recorded, health officials said.

Death is painful, with high fever, severe headache, vomiting, diarrhea and profuse bleeding. Health workers are often among the first to die, and they must take extraordinary precautions to avoid being infected when helping patients, including wearing head-to-toe biohazard suits. The heat inside the suits can be intense, and health workers are counseled not to wear them for more than 15 minutes.

The remaining cases in Conakry are in an isolation ward at the city’s main hospital, Dr. de Clerck said. The current Conakry cases all emanate from an initial infection — five medical workers who treated it, and eight family members — so there is some hope that the disease in the capital can be contained.

“It’s a good sign that the epidemic has not yet spread,” Dr. de Clerck said. “There is no evidence, for now, that it is spreading to other parts of the city. So there is a little bit of hope for the city.”

The World Health Organization is monitoring about 400 people in Guinea for 21 days in order to “break the chain of transmission,” said Gregory Hartl, its chief spokesman. “If they start showing symptoms, we ask them to isolate.”

“The fortunate thing with Ebola is, it’s quite difficult to transmit,” Mr. Hartl said. “You have to touch someone. Fortunately for the greater population, the risks are quite small.”