China says it is screening thousands of passengers at railway stations and airports after the re-emergence of the deadly Sars virus.
The checks for Sars are back
A woman who died last Monday is suspected of having the virus.
Her daughter, who worked at a Sars research lab in Beijing, a co-worker and a nurse have fallen ill.
The lab, in the Center for Disease Control, has been sealed off, China's health ministry said, and workers there have reportedly been quarantined.
Some governments in Asia - the region hardest hit by last year's outbreak of the respiratory virus, which killed about 800 people worldwide - are also braced for new cases.
China's railway stations and airports have been ordered to take the temperatures of passengers in a bid to stop the possible transmission of the disease.
Fever is a key symptom of the illness.
Attention is focused on passengers travelling from Beijing and China's eastern province of Anhui.
The woman who died of suspected Sars came from Anhui. She is believed to have been infected by her daughter, a 26-year-old lab researcher in Beijing, who has a confirmed case of Sars.
The women took several train trips together between Beijing and Anhui, a World Health Organization (WHO) representative told AP news agency - during which they may have exposed other people to the virus.
The outbreak in China comes ahead of a week-long national holiday, Golden Week, which begins on 1 May.
A Beijing nurse who treated the 26-year-old lab worker also now has Sars, the health ministry said.
And a colleague at the lab has a suspected case of the disease.
Sars first emerged in China in November 2002, and the Chinese government was later criticised for being too slow in publicly confronting its spread.
It has vowed to be more open.
The WHO said China had requested help to identify how the virus re-emerged.
"A lab team is being sent to determine what happened," WHO spokesman Dick Thompson told Reuters.
Taiwan and other Asian countries are re-introducing Sars checks
"If it was a laboratory accident,
we have to find out how it happened, why it happened, and how to prevent it from happening in the future."
Meanwhile, other Asian governments stung by the virus last year are taking their own safeguards against a new outbreak.
Staff at Taiwan's international airport are asking passengers to fill out health forms, and are checking passengers for symptoms, said AP.
Singapore has also ordered hospital and airport staff to take extra precautions in handling passengers from China, the agency said.