TORONTO -- British authorities announced Tuesday that another person has died from infection with the new coronavirus as European scientists revealed the new virus easily infects the cells of the airways of the human lung.

In fact, the new coronavirus is as adept at infecting the cells of the upper airways as two cousin viruses -- the one that caused SARS and one that causes common colds, the new study reported. All three are members of the coronavirus family.

Also on Tuesday, the World Health Organization tweaked its case definition for the new coronavirus, in what appears to be an attempt to ensure health officials don't miss mild cases. That comes in the wake of the discovery of a mild infection in a British resident related to the person who just died.

The man who died -- the sixth confirmed fatal infection with this virus -- was part of a cluster of three cases in the U.K. Only one, the first diagnosed, had recently travelled abroad. He is believed to have been infected during his trip, which took him to Pakistan and Saudi Arabia.

The other two family members were infected in the U.K., the first time infections with this virus have been seen to have taken place outside the Middle East. They are the man who died and his female relative, who suffered only a mild illness and has since recovered.

Authorities believe the second and third cases in this family were infected via person-to-person transmission, but it appears there are still unanswered questions about the pattern of spread within this group.

"The second and third cases were infected through human-to-human transmission although the exact route of transmission is still under investigation by the U.K. authorities," the European Centre for Disease Control said Tuesday in a risk assessment on the virus updated to account for the British cluster.

The ECDC statement noted, however, that follow up with approximately 200 health-care workers, family members and other contacts of cases in Europe and the Middle East have not found other instances of secondary spread.

"The (U.K.) Health Protection Agency ... is undertaking intensive follow-up of close contacts of these three recent cases to determine if there have been any further mildly symptomatic or asymptomatic infections," the ECDC statement said.

"Depending upon these findings, case-finding strategies may need to be reviewed, which ECDC is currently considering."

The WHO seemed to be thinking the same thing, releasing late in the day an updated case definition that suggested probable cases could include people who have an acute respiratory illness but who don't necessarily have a fever.

That may be an attempt to ensure that health officials don't overlook mild cases in the search for infections that resemble the high profile ones that have killed half of the 12 confirmed cases and left at least two people in intensive care for weeks or months.

The new WHO case definition also drops a reference to the fact that probable cases should be people whose illness cannot be explained by another infection.

That change may have been prompted by the fact that the first man in this British cluster tested positive for both the new coronavirus and a seasonal flu virus. In some centres if the man's flu test had come back positive before the coronavirus test was ordered, he might not have been spotted as a coronavirus case.

As health authorities scrambled to try to assess the risk of the new virus the new research findings suggest it is already well-adapted to being a human pathogen. The virus is called EMC -- for Erasmus Medical Centre, the Dutch research facility which first identified it -- by some people and NCoV, short for novel coronavirus, by others.

In the new study, Volker Thiel and colleagues tested the new virus in human bronchial cells, comparing the EMC virus with the SARS virus and a human coronavirus called 229E that causes colds. The paper was published in the journal mBio.

The cells were as susceptible to the EMC virus as to the other two and in fact, the new virus multiplied at a faster rate than the SARS virus did in the human cells.

Thiel is with the Institute of Immunobiology is at the Kantonal Hospital in St. Gallen, Switzerland. He also teaches at the University of Zurich. Other scientists on this project are with the University of Bonn Medical Centre, the Helmholtz Center for Infection Research in Braunschweig, Germany and Erasmus Medical Centre in Rotterdam, the Netherlands.

Thiel said the team was not surprised that the new virus could infect the airway cells -- called epithelial cells. But the degree of susceptibility of the cells to the new virus was unexpected.

"We were a bit surprised that it can so easily infect those cells," he said in a telephone interview.

"Usually you think that there is a so-called species barrier when an animal virus gets into a human population. But at least on the epithelium layer, we don't see that."

The new virus was first spotted last June, when a Saudi Arabian man died from an initially unidentified respiratory infection. Since then, cases have emerged sporadically -- some singly, others in small groups. As well, testing on stored samples revealed two people who died in a mysterious respiratory outbreak in Jordan last April were infected with the EMC virus.

All of the infections appear to have a link to the Middle East, with Saudi Arabia, Jordan and Qatar being the three countries from which most cases have arisen.

The source of the new virus is still unknown. As such, there are many unanswered -- and currently unanswerable -- questions about how much of a risk the virus poses to people. No one can say at this point whether it will fade away, continue to trigger the occasional infection, or start to spread easily from person to person.

But the question of whether the virus would need to evolve more to gain the power to infect human lungs does seem to have been answered.

Still, Thiel cautioned that just because the virus can easily infect human lung cells doesn't mean it has all the tools it would need to take off and spread widely among people.

"We have shown that the airway cells can easily be infected. But this does not mean that the virus can easily be transmitted," he said. "I think this distinction is important."

The research may have provided a clue in how to treat infections with the new virus. When the scientists treated the cells with interferons -- signalling proteins that cells release to warn surrounding cells of the presence of an attacker -- the number of infected cells was significantly reduced. Interferons are currently used in the treatment of several viral diseases.