London ? Scientists believe they have found the virus responsible for the mystery illness that has sickened hundreds of people worldwide and are perfecting a test to diagnose it, the World Health Organization announced Saturday.

The advances, by the University of Hong Kong, are considered an important step in slowing the spread of severe acute respiratory syndrome and bring scientists closer to determining how best to treat it.

The progress comes less than a week after the World Health Organization pulled together the talent from 11 laboratories throughout the world in an unprecedented collaboration to hunt down the disease.

“We can be relatively sure that we have now found the causative agent,” said Dr. Klaus Stohr, a WHO virologist who is coordinating the global laboratory network.

“It is the SARS virus. But to which virus family it belongs, we don’t know yet,” he added.

Some researchers believe it is a new type of paramyxovirus, but studies from other labs suggest it may belong to another virus family.

The virus, isolated from the lung tissue of a patient who died from SARS, is the basis for the diagnostic test, which Stohr said could be available to labs by the end of this week and then in the hands of hospital doctors.

Hong Kong residents, some wearing protective masks, wait at a bus stop. Hong Kong remains the area most seriously affected by a mystery illness, with more than half the total cases. The World Health Organization said Saturday that scientists think they have found the virus responsible for the illness.

The test would make it possible for doctors to quickly weed out and isolate patients with the new disease.

The development of the diagnostic test was announced Friday, but experts were cautious because the results had not been confirmed by further experiments. By Saturday its accuracy had been verified in eight more patients and more details were released.

“The consistency of these findings indicates that the test is reliably identifying cases of SARS infection,” the WHO said.

SARS has made 386 people worldwide ill and killed 11 people in the past three weeks. Experts suspect it is linked to an earlier outbreak of an unidentified disease in China.