An outbreak of the deadly Ebola virus is believed to have killed at least 59 people in Guinea and may already have spread to neighboring Liberia, health officials said Monday.

Health workers in Guinea are trying to contain the spread of the disease, which causes severe internal bleeding. In neighboring Liberia, health officials said they are investigating five deaths after a group of people crossed the border from Guinea in search of medical treatment.

“The team is already investigating the situation, tracing contacts, collecting blood samples and sensitizing local health authorities on the disease,” Liberian Health Minister Walter Gwenigale said.

The Ebola virus leads to severe hemorrhagic fever in its victims and has no vaccine or specific treatment. The new cases mark the first time in 20 years that an outbreak of the virus has been reported in West Africa.

Already health workers fear the outbreak could overtax Liberia and Guinea, both deeply impoverished countries with severely limited medical facilities.

Officials in Sierra Leone are also on high alert and have sent medical teams to the border with Guinea, though no cases have emerged so far.

“The Ebola fever is one of the most virulent diseases known to mankind with a fatality rate up to 90 percent,” said Ibrahima Toure, Guinea’s country director for the aid group Plan International.

“Communities in the affected region stretch across the borders and people move freely within this area. This poses a serious risk of the epidemic becoming widespread with devastating consequences,” he said.

The World Health Organization said it is dispatching experts to help ministry officials in Guinea.

Efforts were underway to keep the virus from reaching the capital of Conakry, home to some 3 million people. Panic erupted Sunday amid reports that two of the deaths had occurred in the capital. However, on Monday authorities said that those cases were only under investigation and later proved not to be positive for the virus.

“I usually take a taxi to get to work, but in order to avoid contact with strangers, I’m going to walk instead,” said Touka Mara, a teacher in Conakry.

Authorities said goods in Conakry that had been imported from the affected part of the south were being quarantined as a precautionary measure.

Ebola was first reported in 1976 in Congo and is named for the river where it was recognized. Ebola outbreaks were reported in Congo and Uganda in 2012.

The virus can be transmitted through direct contact with the blood or secretions of an infected person, or with objects that have been contaminated with infected secretions. During communal funerals, for example, when the bereaved come into contact with an Ebola victim, the virus can be contracted, health officials said.