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HONG KONG (Reuters) - Doctors in Hong Kong have successfully treated some patients with the killer respiratory disease using serum from recovered patients, a top doctor and public health expert said on Monday.

This leads doctors to believe that some people with the respiratory disease have been able to produce antibodies, which are found in serum, to fight the illness. Some experts had earlier assumed and feared that this would not happen as the body might not easily develop antibodies.

The disease, known as severe acute respiratory syndrome (SARS), began in southern China in November and later spread to Hong Kong, Singapore, Vietnam, Canada, Germany, the United States, Germany, Taiwan and a few other places.

It has killed 61 people and infected over 1,600 worldwide.

"Facts have proven that in at least 20 of our patients who went through very smooth recovery, their serum has been used to treat very severe sufferers and that has been very successful," said Leung Ping-chung, a professor and surgeon at the Prince of Wales Hospital where the outbreak of the pneumonia-causing virus in Hong Kong was first concentrated.

"The serum of at least 20 patients have been life-saving in the past two weeks for those who were severely ill."

Leung said the presence of antibodies means those who have recovered would have developed some level of immunity against the illness in the future.

COCKTAIL OF ANTI-VIRALS, STEROIDS

Experts expect more than 90 percent of people who get infected to recover. Only between three and five percent die from it, usually those with other serious illnesses.

Apart from using serum from recovered patients, doctors in Hong Kong have been using a cocktail of anti-virals and steroids, which they say have been effective for many patients.

Although previous evidence has pointed to a transmission by droplets through coughing and sneezing, a new wave of infections involving people living in a single residential block has led experts to question if the virus may be airborne or waterborne.

Of the over 600 people infected in Hong Kong, 213 are from a housing estate and of these, 107 are from a single block.

"The drainage may not be good and it may have regurgitated some of the (contaminated) sputum," Leung said.

"We are now examining all possible angles, to see if it is airborne or in the (building's) water mains," a government spokeswoman said.

But however the disease is spread, Leung and other experts in the territory have strongly recommended the use of protection masks, even though there is no guarantee that they will keep people from getting sick.

"Even droplets can pierce through masks. Nothing can ever be foolproof. But given the situation, it will be very stupid not to protect yourself," he said.

Millions of people in Hong Kong are now using masks in public transport and congested areas to protect themselves.