Health authorities in Guinea have reported West Africa's first case of the deadly Marburg virus, a highly infectious virus related to Ebola.
Marburg is carried by bats and has a fatality rate of up to 88 per cent.
It was found in samples taken from a patient who died on August 2 in southern Gueckedou prefecture, the World Health Organisation said.
"The potential for the Marburg virus to spread far and wide means we need to stop it in its tracks," said Dr Matshidiso Moeti, the WHO's regional director for Africa.
The discovery comes two months after the WHO declared an end to Guinea's second Ebola outbreak, which began last year and killed 12 people.
The WHO said the threat from the Marburg virus was high at the national and regional level, but low globally.
"We are working with the health authorities to implement a swift response that builds on Guinea's past experience and expertise in managing Ebola, which is transmitted in a similar way," Dr Moeti said.
The virus is usually associated with exposure to caves or mines where colonies of Rousettus bats live.
Once a human contracts the virus, it is spread through contact with bodily fluids, or contaminated surfaces and materials, the WHO said.
"We applaud the alertness and the quick investigative action by Guinea's health workers," Dr Moeti said.
The case was detected in a village in a forested region close to the country's borders with Sierra Leone and Liberia.
The patient's symptoms date back to July 25, the WHO said.
After being initially treated at a local clinic and tested for malaria, the man died "in the community", the WHO said.
Samples taken during a post-mortem examination tested negative for Ebola, but positive for Marburg.
Ten WHO experts, including epidemiologists and socio-anthropologists, are supporting national health authorities.
The emergency response includes risk assessment, disease surveillance, community screening, clinical care, infection control and logistical support, the WHO said.
Cross-border surveillance has also been increased so potential cases can be detected quickly, it said.
Three of the patient's family members and a healthcare worker have been identified as high-risk close contacts and are being monitored, the WHO said.
Investigations are under way to identify the source of the infection and any other possible contacts.
Previous outbreaks and sporadic cases have been reported in South Africa, Angola, Kenya, Uganda and the Democratic Republic of the Congo.
But this is the first time the virus has been detected in West Africa.
The disease begins suddenly, with symptoms such as high fever, severe headache and discomfort.
Fatality rates of between 24 per cent and 88 per cent were reported in previous outbreaks and depended on the virus strain and case management, the WHO said.
Although there are no approved vaccines or antiviral treatments, oral or intravenous rehydration and treatment of specific symptoms improve the chances of survival, it said.