A PREVIOUSLY unknown virus that killed four of the five people it struck in an outbreak in South Africa last year has been identified as part of a family of viruses humans can catch from rats.

The virus, named Lujo, is an arenavirus that over nine days caused rash, fever, muscle pain, diarrhoea, severe bleeding, vomiting, organ failure and death, said Nivesh Sewlall, who treated the first patient at Johannesburg's Morningside MediClinic Hospital. He reported the findings at an infectious disease conference in San Francisco yesterday.

Lujo appears more dangerous than other arenaviruses and related hemorrhagic fever syndromes, with the exception of Ebola and Marburg, which have similar fatality rates of about 80 per cent. Outbreaks of illness from these viruses have been sporadic and not widespread, but the World Health Organisation has said population growth into remote areas and urbanisation have led to new diseases emerging more quickly in recent years.

''It wasn't the outer jungle, this was suburban Lusaka,'' said Dr Sewlall, a pulmonary and critical care physician at Morningside. ''There are parts of Central Park that are more rural,'' he said, referring to the park in New York.

Scientists do not know where the pathogen came from, although humans usually catch other arenaviruses such as Lassa fever by inhaling dust contaminated with rodent droppings, according to the US Centre for Disease Control and Prevention in Atlanta.