Disease experts said Thursday the SARS virus appeared to be just as hardy in its 15th victim as its first one, suggesting its ability to spread wasn’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 did not appear to have happened and that scientists estimated the SARS virus had 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 still are 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.”

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.