Leaky sewage pipes and bathroom ventilation fans carried contaminated droplets through parts of a Hong Kong (searchapartment complex, causing one of the world's worst outbreaks of SARS (search), World Health Organization (searchinvestigators said Friday.

More than 300 people came down with the illness at the Amoy Gardens (search) apartment complex in late March, and 35 people died. The speed of the infection amazed health experts, who at the time believed the disease was spread mainly by person-to-person contact.

A report written by a team of WHO investigators blamed an "unlucky" combination of circumstances -- a patient with diarrhea, seeping pipes and drafty air shafts.

"It's just an accumulation of events," team leader Dr. Heinz Feldmann told a news conference. Feldmann said there was no way to guarantee against a repeat but that another such outbreak seems "unlikely."

When the WHO investigators went to Amoy Gardens to collect samples, they found no live coronaviruses -- the family of virus believed to cause severe acute respiratory syndrome -- and no remaining genetic material from the virus, Feldmann said. The WHO's findings largely confirm an earlier report by Hong Kong officials.

The WHO team is still conducting lab tests on samples collected from another housing development, the Tung Tau Estate, that suffered a minor outbreak. Feldmann said preliminary findings showed the sewage system did not appear to be the cause, but he did not elaborate.

The disease was brought to the Block E building of Amoy Gardens by a sick man visiting his brother, the Hong Kong government said earlier. The man had diarrhea and others who caught SARS in the building also developed diarrhea, spreading the virus in the sewage system.

Droplets containing the virus apparently got into some apartment units through dried-out drain traps -- the U-shaped pipes that are supposed to keep gases and waste from coming up back up. Exhaust fans in the bathrooms sucked the droplets into apartments, the WHO report said.

The exhaust fans could also have moved contaminated droplets into a light and air shaft, where wind moved it into other apartments through open windows.

Feldmann said there was no evidence the virus itself was airborne, but small droplets can travel up to five feet through the air, perhaps further if helped by a strong wind.

A break in a pipe shut down the water used to flush toilets at one point, which may have trapped some infected sewage in place and allowed the virus to multiply, the team said.

Feldmann said an earlier outbreak of SARS at Hong Kong's Metropole Hotel appears to have been caused by close person-to-person contact.

An ill mainland Chinese medical professor visiting the Metropole in late February infected 16 people who spent time on the ninth floor. They in turn spread SARS throughout Hong Kong and to three other places that suffered fatal outbreaks: Vietnam, Canada and Singapore.

Feldmann said the investigators did not find any problem with the sewage or ventilation systems at the hotel.

"We think people were safe in their rooms," he said.

People probably caught the virus in common areas like hallways, he said.

SARS has infected 1,703 people in Hong Kong and killed 234.

In China, officials on Friday began to punish people for violating SARS-related restrictions. One woman was given a one-year sentence for leading protesters who vandalized a building being turned into a quarantine center.

Two school principals in the northeastern Chinese city of Harbin were fired for holding entrance tests in defiance of orders to postpone them, the official China Youth Daily reported.

Singapore remained hopeful that its outbreak could be declared over, after 15 of the more than 40 people showing SARS symptoms at its largest mental hospital tested negative for the virus.

Japan, which has had no reported cases of SARS, announced a $13 million emergency aid package to help China fight the disease. It also will donate 20,000 protective suits.

SARS has killed at least 605 people worldwide and infected more than 7,700 since emerging in southern China in November.

Air travelers have been instrumental in its spread to more than 30 countries, and Asian airport officials meeting in the Philippines on Friday adopted more measures to contain the illness.

The officials agreed to have standardized health declaration cards for departing passengers by June 15 and temperature screening for departing travelers implemented by Aug. 15.

Representatives from the 10-member Association of Southeast Asian Nations were joined by airport officials from China, Japan and South Korea. They agreed that passengers suspected of SARS would be quarantined and placed under medical care but that they would not be denied entry into any country in the region.

Ireland, which is hosting next month's Special Olympics, on Thursday told delegations from China, Hong Kong, the Philippines, Singapore and Taiwan that they should not attend the mid-June event. Organizers condemned the decision as discrimination against handicapped people.