Health authorities have been worrying for months about the reappearance of Severe Acute Respiratory Syndrome in winter.
The disease is an atypical pneumonia, with the same symptoms, including fever and dry cough, as several other respiratory diseases.
"Some of these diseases may also give rise to atypical pneumonia. It is likely that numerous other suspected (SARS) cases will be reported over the coming weeks," the WHO said on its website.
The only confirmed case so far has been in southern China. However, the 32-year-old TV producer has recovered and left hospital.
A 20-year-old waitress in Guangzhou, capital of Guangdong province, is also suspected of having contracted SARS.
Two suspected cases in Hong Kong were cleared on Thursday with results of a third case awaited.
"Some of these diseases may also give rise to atypical pneumonia. It is likely that numerous other suspected (SARS) cases will be reported over the coming weeks"
World Health Organisation
The WHO has sent a team of four doctors to Guangzhou to investigate the disease, particularly its transmission.
A spokesman said their investigation would be sweeping, with all possibilities being considered.
"Really it's a free-for-all," team spokesman Roy Wadia said, referring to the scope of their research. "There's very little that's not on the table."
SARS first appeared in southern China in late 2002 and killed about 800 people worldwide last year, nearly 350 of them in China.
A Guangzhou health official said he did not believe SARS would re-emerge on the scale of last year.
"I do not think the confirmed case means that SARS will return on a large scale," Zhong Nanshan, head of the Guangzhou Respiratory Illness Research Institute, was quoted in the media as saying.
"To say it will trigger the huge spread of SARS is absurd."
The China Daily newspaper said the country had learnt its lesson from last year, when it first covered up the extent of the disease.
China's prompt response this time
has come in for all-round praise
"The prompt responses to the two cases shows that we have learnt from last year's outbreak," it said in an editorial. "Transparency breeds confidence."
The latest cases have been linked to a coronavirus very similar to one found in civet cats, a weasel-like animal prized as a delicacy in southern China and sold in crowded markets.
With the re-emergence of SARS, the government banned sale of the animals and has been carrying out a mass cull.
The SARS scare is emerging just before the Lunar New Year holidays, when an estimated 1.89 billion journeys are expected to be made by rail, road, ship and air around China and in the region.
Dr Julie Gerberding, head of the US Centres for Disease
Control and Prevention, said much work remained to identify the precise source of SARS, although experts believe it was an animal virus that mutated to easily infect humans.
"We may not ever be able to identify it," Gerberding said.
"What we are seeing in China is a very vigilant health system that is doing exactly what it should be doing," Gerberding said.
But French actress turned animal rights activist Brigitte Bardot protested against the civet cat cull.
"The eradication methods these animals are put through are unacceptable," Bardot said in an open letter to Chinese President Hu Jintao.
"No scientific research has yet identified which species is the first to have caught the virus."