The SARS virus that infected hundreds of people in a 33-story Hong Kong apartment tower probably spread in part by traveling through bathroom drainpipes, officials said yesterday in what would be a disturbing new confirmation of the microbe's versatility.

The possible explanation for what has been one of the most baffling and worrisome outbreaks in the epidemic indicates that the virus can be transmitted in ways other than close person-to-person contact.

"The possibility that the virus could by aerosol move through a vertical pipe through other pipes . . . into the air and affect so many people, that's not comforting," said Klaus Stohr, who is leading the World Health Organization's scientific efforts against SARS -- severe acute respiratory syndrome.

Stohr stressed, however, that even if the virus can spread through plumbing or in other ways, the pattern of the epidemic so far indicates that would occur only rarely.

"If this would significantly contribute to the transmission of the virus, the epidemic would look different. It would spread faster. We'd have many more cases that we could not link to a SARS patient," Stohr said.

Nevertheless, evidence of the virus has been found in almost every body fluid that has been tested, including blood, feces, urine, saliva and even tears, Stohr said. Moreover, other members of the coronavirus family can be very rugged, surviving at freezing temperatures for as long as a year, in direct sunlight for 10 hours and at room temperature for two weeks, Stohr said. "We know that some coronaviruses are quite resistant," Stohr said in reference to the type of virus now reported as the cause of SARS.

The SARS virus appears to spread primarily in droplets expelled when an infected person sneezes or coughs. But when 321 people became infected in the Block E building in the Amoy Gardens apartment complex in Hong Kong, health authorities said that kind of person-to-person transmission could not account for it. The outbreak prompted officials to evacuate the building and quarantine its residents in camps in the countryside.

Hong Kong health officials then launched an intensive investigation to try to determine how the virus was spreading. They swabbed elevator buttons and other objects for traces of the virus, examined the building's ventilation and plumbing systems, and caught and tested rats, mice and cockroaches.

Based on the pattern of when people started getting sick, investigators traced the outbreak to a 33-year-old man from mainland China who regularly visited his brother in the building. The man suffered from liver disease and was being treated at the Prince of Wales Hospital in Hong Kong, which has been hit by one of the largest SARS outbreaks.

The man started to develop SARS symptoms on March 14 and visited his brother on that day and again on March 19. He had diarrhea at the time and used his brother's bathroom. The man's brother and sister-in-law and two nurses who treated him at the hospital subsequently developed SARS.

"When you look at the onset of illness, we believe that the patient was the initial source of infection," said Yeoh Eng-kiong, Hong Kong's secretary for health, welfare and food, at a briefing.

An unusually large proportion of those affected -- two-thirds -- developed diarrhea, he said.

Each Amoy Gardens building has eight vertical pipes that collect waste from toilets, bathtubs, sinks and floor drains, and each drain has a U-shaped trap designed to prevent foul odors, insects and fluids from coming back up. The traps must be filled with water to work, however, and often were not.

"Interviews with Amoy Gardens residents revealed frequent complaints about foul smells in the bathrooms," Yeoh said. One test showed air spewed from a drain when an exhaust fan was on.

"When the bathroom was in use, with the door closed and the exhaust fan switched on, there could be negative pressure to extract contaminated droplets into the bathroom," he said. "Contaminated droplets could then have been deposited on various surfaces such as floor mats, towels, toiletries and other bathroom equipment."

Another test found a crack in a sewer vent pipe on the fourth floor, which could also have allowed sewage to spread into apartments through the building's light shaft.

While no virus was detected in air and water samples collected in the building, virus was detected in a toilet in the apartment of a resident who fell ill, Yeoh said.

No infected rodents were found, but the building was infested with cockroaches. "Coronavirus was also detected in the pest droppings and in some instances on the surfaces of the cockroaches," indicating the insects could have physically carried virus around the building, he said. "They were just passive, mechanical carriers."

Yeoh stressed that "there is no evidence to suggest that the disease is transmitted by the waterborne route, or by infected dust aerosols" or by virus hanging in the air.

Based on the findings, investigators concluded that the first patient infected a small number of residents in the building, who then went on to infect other residents through the sewage system and by contaminating communal facilities such as elevators.

"So there is a whole series of methods of infection. One is through the sewage -- through droplets -- the second is through person-to-person contacts, and the third is through environmental contamination," he said.

The building has subsequently been thoroughly disinfected, and residents were advised on how to keep their drains sterile and operating properly.

The report came as the number of suspected cases of SARS increased to 3,389 in 25 countries, according to WHO. Australia and Mongolia each reported their first three cases. India reported its first case. At least 165 people have died worldwide.

A medical staffer takes the temperature of a passenger at the Hong Kong Airport to prevent the spread of SARS. At least 165 people have died.