Hong Kong ? Adding to fears that a deadly flu-like illness is being spread by air travelers, Hong Kong officials said Tuesday nine tourists apparently came down with the deadly disease after another passenger infected them on a flight to Beijing.

The World Health Organization insisted air travel is safe but said its scientists were investigating each case to make sure the disease was not spread through ventilation.

In recent weeks severe acute respiratory syndrome, or SARS, has spread beyond hospitals, where dozens of health care workers became infected, into at least one workplace, to air travelers and some schools have been closed as a precaution.

Hong Kong officials said the nine tourists became sick after a mainland Chinese man with SARS infected them on a March 15 Air China flight to Beijing.

If SARS can be more easily spread through the air — rather than by close contact with infected people who cough or sneeze — it could force travel and other restrictions to contain the disease.

“We would want to be sure that it was people sitting next to that person and not the ventilation system in the airplane which was spreading the disease,” said Dr. David Heymann, head of communicable diseases at WHO. “We have no evidence of the latter right now.”

For one thing, he said, health investigators have followed thousands of passengers who flew with SARS-infected travelers and did not become sick.

However, he said that if they find there are cases that did not involve close contact with someone sick or at high risk, “we will then be very concerned that this might have become airborne.”

The airplane cases seem similar to how the disease got its start here — from one hotel guest who spread it to six strangers staying on the same floor. One expert theorized it might have spread through the air-conditioning system.