The so-called "Lujo" virus infected five people in Zambia and South Africa last fall. Four of them died, but a fifth survived, perhaps helped by a medicine recommended by the scientists.
It's not clear how the first person became infected, but the bug comes from a family of viruses found in rodents, said Dr. Ian Lipkin, a Columbia University epidemiologist involved in the discovery.
"This one is really, really aggressive" he said of the virus.
A paper on the virus by Lipkin and his collaborators was published online Thursday on in PLoS Pathogens.
The outbreak started in September, when a female travel agent who lives on the outskirts of Lusaka, Zambia, became ill with a fever-like illness that quickly grew much worse.
She was airlifted to Johannesburg, South Africa, where she died.
A paramedic in Lusaka who treated her also became sick, was transported to Johannesburg and died. The three others infected were health care workers in Johannesburg.
Investigators believe the virus spread from person to person through contact with infected body fluids.
"It's not a kind of virus like the flu that can spread widely," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which helped fund the research.
The name given to the virus -; "Lujo" - stems from Lusaka and Johannesburg, the cities where it was first identified.
Investigators in Africa thought the illness might be Ebola, because some of the patients had bleeding in the gums and around needle injection sites, said Stuart Nichol, chief of the molecular biology lab in the CDC's Special Pathogens Branch. Other symptoms include fever, shock, coma and organ failure.
Genetic extracts of blood and liver from the victims were tested at Columbia University in New York, and additional testing was done at CDC in Atlanta. Tests determined it belonged to the arenavirus family, and that it is distantly related to Lassa fever, another disease found in Africa.
The drug ribavirin, which is given to Lassa victims, was given to the fifth Lujo virus patient - a Johannesburg nurse. It's not clear if the medicine made a difference or if she just had a milder case of the disease, but she fully recovered, Nichol said.
The research is a startling example of how quickly scientists can now identify new viruses, Fauci said. Using genetic sequencing techniques, the virus was identified in a matter of a few days - a process that used to take weeks or longer.
Along with Fauci's institute, the National Heart, Lung, and Blood Institute and Google also helped fund the research.