Maintenance were disinfecting three schools in Queens yesterday, where hundreds of students were sent home sick after a swine flu outbreak that left an assistant principal hospitalised in critical condition on a breathing tube.

Meanwhile, health investigators are puzzling out why swine flu has spread quickly through a few schools, but slowly elsewhere. Mayor Michael Bloomberg announced the school closures Thursday evening, saying four students and the assistant principal at the Susan B. Anthony middle school in Hollis have documented cases of swine flu.

The mayor said the assistant principal may have had pre-existing health problems, but yesterday relatives denied that.

It isn't unusual for flu cases to ebb and surge as the virus moves through a population during flu season. The virus tends to disappear as the weather gets warmer and more humid.

Dr Isaac Weisfuse, a deputy commissioner of the health department, said investigators are trying to learn more about why the disease has spread erratically.

Schools are a good incubator for illness in general, he said, because space is tight and youngsters often don't practise the best hygiene.So far, the virus has not proved to be more infectious or deadly than the seasonal flu.

At least five schools in the city were closed in April, but all have since reopened. Officials say the students who have fallen ill in this latest surge of illness appear to be experiencing mild symptoms, similar to routine flu.

Bloomberg said the three schools with more than 4,000 students altogether - would be closed for at least a week because "there are an unusually high level of flu-like illnesses at those schools".

"There are documented cases of H1N1 flu at one of them," the mayor said, using the formal name for swine flu.

New York City's first outbreak occurred when hundreds of teenagers at a Roman Catholic high school in Queens began falling ill following the return of several students from vacations in Mexico, where the outbreak began.