A haemorrhagic fever that killed two people in the Democratic Republic of Congo in 2009 and left a third person seriously ill was caused by a novel virus, researchers said.
The newly-found microbe is likely to exist naturally among insects or animals but may also be spread by human contact, they said.
It has been called Bas-Congo virus, or BASV, after the province in southwestern DR Congo where the three cases occurred, they said in a genetic study of the pathogen.
"These are the only three cases known to have occurred, although there could be additional outbreaks from this virus in the future," said Charles Chiu of the University of California at San Francisco.
BASV is unlike any of the other viruses known to cause haemorrhagic fever, a group of diseases that causes severe fever and muscle pain, sometimes ending in organ failure or unstoppable bleeding.
Called a rhabdovirus, it is genetically closer to the types of virus that cause rabies, according to their study, published on Thursday in the journal PLoS Pathogens. In fish, rhabdoviruses can cause fatal bleeding called haemorrhagic septicaemia, previous research has found.
The outbreak occurred in mid-2009 when a 15-year-old boy in a remote village called Mangala fell ill with a bleeding nose and gums, and bloody vomit, and died within three days, the paper said.
A week later, a 13-year-old girl who lived in the same neighbourhood and attended the same school fell sick with the same symptoms, also dying within three days.
The third case, who survived, was a male nurse who had cared for the girl.
He was transferred to a hospital at the river port city of Boma, where doctors took samples which led to the viral discovery.
Tests of other people who had been in contact with the third patient found that a caregiver who had treated him in Boma also had BASV virus in the blood, but had not fallen ill.
"What this suggests is that the disease may be transmissible from person to person, though it's most likely to have originated from some other source," said Nathan Wolfe, a co-author who heads a California health company, Global Viral Forecasting.
"The fact that it belongs to a family of viruses known to infect a wide variety of mammals, insects and other animals means that it may perpetually exist in insect or other 'host' species and was accidentally passed to humans through insect bites or other means."
Other microbes that cause haemorrhagic fever include the Ebola, Lassa and Crimean-Congo viruses.
An ongoing outbreak of Ebola in the DR Congo may have killed up to 33 people, the health ministry in Kinshasa said on Thursday. In Uganda, 17 deaths occurred from Ebola in an outbreak in July and August in the west of the country near the Congolese border.
The researchers said they had developed an antibody test for BASV that should help diagnosis in future outbreaks.