Beijing ? Beijing has not explained how half of its SARS patients caught the virus, hindering efforts to slow the highly contagious illness in the world’s most populous nation, the World Health Organization said Saturday.

The WHO announcement came as Taiwan reported its biggest one-day jump in SARS cases, 23, and four more deaths. The virus is surfacing in new, unexpected places in Taiwan, according to the WHO, and the U.S. State Department said it was allowing its nonessential personnel to leave the island.

China reported five more deaths and Hong Kong two, boosting the worldwide SARS death count to at least 526. More than 7,200 people have been infected in more than 25 countries, including 85 new cases in China.

China has reported more than 4,800 SARS cases.

The WHO still is trying to figure out why Beijing does not have the data, or so-called “contact reports,” that help trace how a patient became infected with SARS, or severe acute respiratory syndrome, WHO spokeswoman Mangai Balasegaram said.

“Right now the situation is that we have a whole load of people, and we don’t know where they got the disease,” she said. “The epidemic might be flying off in one direction, and you might not know about it.”

Balasegaram said the lack of information might have to do with bad reporting, some hospitals inadvertently failing to file complete reports or the changing nature of the illness itself.

“It’s just a whole load of unknowns,” she said.

At a news conference Friday, Beijing officials acknowledged that about half the city’s nearly 2,200 cases have not been traced but they did not directly explain why.