Disease experts said Thursday the SARS (search) 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 (search)'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 health experts talking about the need for more quarantine space.

U.S. Centers for Disease Control and Prevention (search) 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. A charter flight arrived there Friday morning to pick up the official.

The CDC would not identify the epidemiologist but said he would return home and enter an Atlanta hospital this weekend. Taiwan's health chief said an X-ray did not indicate pneumonia and the doctor did not test positive for SARS. However, he could be in the very early stages of the disease.

Taiwan reported eight new SARS deaths and 65 more cases, its biggest one-day jump, bringing its total to 483 cases and 60 deaths. That gives the island the third-highest toll after mainland China and Hong Kong.

Su Yi-jen, chief of Taiwan's Center for Disease Control said Taiwan was, "at the peak of the new wave, and we're at the stage when we're about to come down."

The World Health Organization announced it is launching a fund to help Asian nations combat the virus. The U.N. agency said it is seeking $200 million to boost SARS surveillance and analysis in China and other hard-hit nations.

The WHO said it hoped businesses — particularly those with operations or markets in Asia — would donate half the money, with the rest coming from governments.

"Despite the rapid and effective response from partners around the world, SARS exposes fundamental weaknesses in global health infrastructure," Dr. Jong-wook Lee, WHO's newly elected director general, said in a statement.

"This new fund and other innovative initiatives like it will help prepare the world to respond to future emerging diseases."

In Beijing, some students returned to classes after a month off because of the epidemic, but the city reported a slight jump in new cases.

While normalcy has returned to much of the city, new figures indicated the battle against the disease is not over. China announced 26 new SARS cases — the largest daily increase this week — and four deaths.

In Sweden Thursday, U.S. Health and Human Services Secretary Tommy Thompson said U.S. efforts to counter the threat of biological terrorism has made the United States better prepared to respond to the SARS epidemic.

"The bioterrorism preparedness has certainly got us better prepared to respond not only to bioterrorism attacks, but also to a public health infection epidemic like SARS or any other pandemic flu that may come," he told reporters during a two-day visit to Sweden.