We still don't know. Severe acute respiratory syndrome (Sars) came out of nowhere, has infected over 2,300 people living in 13 countries and killed almost 80 (as of yesterday), and there is no treatment. That makes it alarming, but not a modern plague.
Those who have died, including the World Health Organisation infectious diseases expert Dr Carlo Urbani who first raised the alarm, have mostly been over the age of 40 with underlying conditions that made them less capable of fighting off the viral onslaught. These health conditions have included coronary heart disease or liver damage. Around 90% of those who are infected start to recover after six or seven days with good nursing, even though antibiotics and antiviral drugs are ineffective.
Experts are pretty sure now that Sars is a form of atypical pneumonia and not the long-predicted flu pandemic, which would spread faster. They think it is caused by a new strain of the corona virus quite unlike any we have known in animals or humans until now, although it probably mutated from a known virus in a human or jumped species from an animal. Some of these viruses are aggressive. It is also possible that a second virus, a paramyxo virus isolated from some of the patients, could play some part.
The spread of the infection is worrying, although careful containment measures have stopped its transmission in Vietnam and are expected to do the same in Singapore and Canada. But Hong Kong (above) has put an entire high-rise housing block under curfew after over 100 residents were admitted to hospital, raising questions about whether the virus can be transmitted through shared pipes. For the most part, however, transmission appears to occur from heavy water droplets face to face.
Alerts have gone out to the world's airlines and WHO experts advise moving if you are within two seats of somebody travelling from a high-risk country who is coughing or looks ill.
The hunt is on for a drug to treat Sars and, since those who have survived have done so by producing antibodies to the virus, a vaccine may well be possible.