Disease experts said Thursday the SARS virus appears to be just as hardy in its 15th victim as its first one, suggesting its ability to spread isn't weakening.
The virus' robust nature indicates it is well-adapted to reproducing inside the human body, health experts said.
There had been hope that, like some viruses, this one might lose its punch over time, mutating and weakening. However, Dr. David Heymann, the WHO's chief of communicable diseases, said that does not appear to have happened and that scientists estimate the SARS virus has passed through chains of up to 15 people.
Heymann said he was not speaking of the virulence of the disease -- only its ability to continue infecting people over time.
He gave an example of how some viruses weaken, citing the human monkeypox virus, similar to smallpox, but less deadly, and sometimes seen in Africa. Monkeypox "comes out of an animal into humans. It causes disease and it transmits maybe to one or two generations, but by the time it has gone through that, it never transmits further," Heymann said.
SARS, however, does not appear to be weakening. WHO experts are still trying to determine the maximum number of people who have become infected in a single chain. Fifteen is as far as they got.
Using an incubation period of about 10 days, scientists calculated about three infections from one SARS case in a month. Tracing cases over five months' time, "that's how we come up with the 15," Heymann said. "The hypothesis still remains that this all came from one person, so it has passed through many, many people on it's way out through the world."
"We think it's a virus that will continue to transmit in humans unless we interrupt its transmission by isolation and containment," Heymann said.
SARS is believed to have first surfaced in November in the southern Chinese province of Guangdong. Scientists have traced some infections today back to one original case in the province, but more definitive studies are being done on its spread.
More than 8,000 people have been infected worldwide, with a death toll of 684. In recent weeks, the island of Taiwan has been hardest-hit, with 20,000 people in quarantine and experts talking about the need for more space.
U.S. Centers for Disease Control and Prevention officials said Thursday that doctors' difficulty in diagnosing SARS cases may have led to hospital outbreaks in both Taiwan and Singapore.
In Taiwan's capital of Taipei, a CDC expert with SARS symptoms was being whisked back to the United States, officials said.
The CDC would not identify the epidemiologist but said he would enter an Atlanta hospital.