All kindergartens and junior schools in Hong Kong have closed early for their Easter holiday, after a flu-like illness killed three children.
The government described the move as a "precautionary measure" to ease parents' fears.
Almost 200 people had been affected, officials said. At one school, some 30 students showed flu-like symptoms.
Experts are working to identify the virus and assess whether it poses a broader threat.
But health officials played down fears of a return of the Sars (Severe Acute Respiratory Syndrome) virus.
World Health Organisation spokesman Peter Cordingley said it appeared to be "regular seasonal flu", the Associated Press news agency said.
The kindergartens and junior schools will be closed for two weeks, affecting more than half a million children.
Hong Kong's health chief, York Chow, said the decision was made as the high rate of flu-like infections was expected to continue for weeks.
"We hope such precautionary measures will help reduce the cross-infection of the flu virus in schools and the community," he said.
The closure would allow schools to be disinfected, he said.
The move came after seven-year-old Law Ho-ming was admitted to hospital in a semi-conscious state, suffering from fever and flu. He died on Tuesday from encephalitis - swelling of the brain.
Five of his fellow students from the Ho Yat Tung Primary School remain in hospital, and more than 30 others are displaying flu-like symptoms.
Concerns have also been raised about two other recent child deaths attributed to flu variants.
Official records show that people in 25 other schools, a hospital ward and a disabled residential home are suffering from outbreaks of flu.
The school closure is the first caused by illness since 2003, when almost 300 people died in the territory from Sars.
The government has set up an expert team led by a University of Hong Kong microbiologist to identify the illnesses in question and assess the risk.
One scientist said it was too soon to tell if the current outbreak of flu was dangerous or not, reports the BBC's Vaudine England from Hong Kong.
Hong Kong's recent history of dangerous disease outbreaks is feeding fears now, but the scientists insist evidence to support those fears does not yet exist, our correspondent adds.
This is potentially very worrying indeed. I am due to travel to HK on Friday with 2 young children, not sure what the implications for the welfare of my family and travel arrangements might be.
Richard, North Yorkshire
There not only happen in Hong Kong, I think it happen in world wide. Many people including me got virus infection over a month, it not over yet. U.K.
This is very worrying. Looking from another country, not actually seeing what is happening in front of your face just causes fear to swell up within you.
School has also been closed down, and it appears the most students are partying and happier then ever. Primary school is scheduled to return in 2 weeks.
Quinton S., Hong Kong
I applaud the HK public health department. They seem to have moved relatively quickly to prevent more deaths. Perhaps an alert system should be set up at every school, a liaison between parent, family doctor and the health department that closely monitors any child who gets sick regardless of the circumstances. This might be one way to avoid a major pandemic.
I have a family in Shenzhen and my kid travels to HK for school everyday. I just called them and heard they do not have to attend school for two weeks. He was told to wear mask outside and not to play with other kids.
It's clear and silent as the children do not need to wait for the school bus downstaire
Simon Tsang, Hong Kong
I live in Hong Kong with my wife and two children (4 & 2), and they are now both off school. This seems an excessive reaction by the government, and my kids will surely play and be in contact with other children over the next few weeks, just not in a school environment. My wife is livid as she works from!
Aidan McNeil, Hong Kong, China