* Death brings worldwide death toll from new virus to six
* Previously unknown coronavirus emerged in Middle East
* Health officials say risk to public remains low (Adds quotes from health officials, background, latest research)
By Kate Kelland
LONDON, Feb 19 (Reuters) - A British man infected with a new virus from the same family as SARS has died, health officials said on Tuesday, bringing the worldwide death toll from the previously unknown disease to six.
The virus, called novel coronavirus or NCoV, was unknown in humans until it emerged in the Middle East last year. There have been 12 confirmed cases worldwide - including in Saudi Arabia, Jordan and Britain - and half the patients have died.
"This patient had an underlying condition that may have made them more susceptible to respiratory infections," the Health Protection Agency said in a statement announcing the death of one of four people in Britain with the NCoV infection.
NCoV was identified when the World Health Organisation issued an international alert in September 2012, saying a completely new virus had infected a Qatari man in Britain who had recently been in Saudi Arabia.
Since then 12 cases have been identified, including a cluster of cases reported last week in a family in Britain.
The HPA said it had followed up more than 100 people who had close contact with cases in this family cluster.
"Besides the identified secondary cases, all tests carried out on contacts to date have been negative for the novel coronavirus infection," it said.
Coronaviruses are a family of viruses that includes those that cause the common cold as well as the one that caused SARS - which emerged in China in 2002 and killed about a 10th of the 8,000 people it infected worldwide.
Scientists are not sure where the new virus came from, but say one possibility is it came from animals. HPA experts say preliminary scientific analysis suggests its closest relatives are bat coronaviruses.
In one of the first published papers on NCoV, scientists said on Tuesday that while it might have jumped from animals, it was already well adapted to infect humans and could penetrate the lining of passageways in the lungs and evade the immune system as easily as a cold virus could.
This shows it "grows very efficiently" in human cells and suggests it is well-equipped for infecting humans, said Volker Thiel of the Institute of Immunobiology at Kantonal Hospital in Switzerland, who led the study.
John Watson, the HPA's head of respiratory diseases said the recent cluster of cases, two of which had no recent history of travel to the Middle East, provided "strong evidence of human-to-human transmission" of the new virus "in at least some circumstances".
Ian Jones, a professor of virology at Britain's University of Reading said currently NCoV "does not appear threatening".
"It is related genetically to SARS but only distantly and the other known human coronaviruses cause only mild disease," he said in an emailed comment.
He added, however, that like bird flu, it may in future have the potential to spread more widely "but not without acquiring multiple changes".
Watson said the cluster of British cases offer an opportunity for researchers to find out more about the characteristics of the infection in humans and risk factors for contracting it.
"The risk of infection in contacts in most circumstances is still considered to be low and the risk ... to the general UK population remains very low," he said. (Editing by Alison Williams)