PARIS -- The SARS virus, the fatal respiratory illness confounding doctors and researchers, could become the first serious new disease of the century and reach pandemic proportions in a world where people are constantly on the move.

With the exception of aids, most of the worrying diseases that surfaced in the last century, like the ebola virus, never really posed a danger to public health at an international level.

This was mainly because transmission of the disease between humans was never optimal or because those who fell ill were simply too sick to travel.

But global airline travel has provided SARS with an ideal vector.

A month after the World Health Organisation (WHO) sounded the international alert over severe acute respiratory syndrome, which has killed more than 120 people among more than 3,000 reported cases, there is still serious concern among the experts despite the measures taken to contain it.

The WHO is not hiding fears that the virus, which begins with high fever, breathing difficulties and a dry cough, could spread to Africa. "The mortality rate is around four percent, so 96 percent of people pull through all right," said WHO doctor Isabelle Nuttall, but she admits that "rate depends entirely on the kind of treament that is available to people."

Another reason for concern is the number of hospital staff who have contracted SARS in the areas where it has spread most rapidly, like China, Hong Kong and Singapore, but also in Canada.

In trying to trace back the source of the outbreaks, scientists have discovered a number of "super-contaminators", people who for some unknown reason spread the virus really well.

In the southern Chinese province of Guangdong, where SARS apparently broke out in November, one person alone could have spread the disease to more than 100 others.

Do these "super-contaminators" carry or spread particularly large quantities of the virus? Do other factors amplify its spread? Do these people carry another strain? All unanswered questions the experts are posing.

According to David Heymann, executive director for transmissable diseases at the WHO, no one is even sure whether someone without symptoms can spread SARS.

However SARS, for which there is no vaccination, is nowhere near as contagious as influenza, which kills around 20,000 people a year in the United States and from 250,000 to half a million worldwide annually.

SARS is spread most of the time in simple face-to-face human contact by saliva or other body fluids from a person who has the illness.

The first simple step in protection against it, as with most diseases, is to wash your hands. Certain enigmas surrounding the centers of the two main outbreaks in Hong Kong are troubling the experts. (AFP)