Nine blood donors have received unwelcome news after giving blood to the Red Cross: they carry a rare bug that puts them at risk of colorectal cancer.
Two have already developed the disease - the second most common cancer in Hong Kong - and five have benign polyps that can turn cancerous over time.
Only two tests were completely clear.
The nine are among 16 people found with Streptococcus bovis bacteremia in a backtrack study by the Red Cross. The others could not be contacted.
Red Cross Blood Transfusion Service consultant Dr Lee Cheuk-kwong said the rare bacteria had been found to be closely linked to polyps - growths on the lining of colon or rectum - and colon cancer.
One of those found with polyps, a 44-year-old regular donor giving his name only as Chiu, said he had been "very shocked" .
"Now I am glad so that I can pay extra attention to my health. The doctor told me one of the three polyps has developed an abnormality so it may not be long before it turns into a cancer," Chiu said.
Under normal procedures all donated blood is tested, with the result expected in 24 hours.
From 1998 to last year, the centre received more than a million units of platelets in which it detected 16 cases of the bacteria.
"All questionable platelets were disposed of immediately and never used in any transfusion," Lee said.
The nine infected donors who could be traced were unaware of the condition and had never displayed any symptoms. In some cases the bug can cause endocarditis - an inflammation of the inner layer of the heart - septicemia and meningitis as well as colon cancer.
Lee said the bacteria was transmissible but the amount in the blood was usually too low to be passed on in usual contact.
"If blood tests positive for an infectious disease, after the result is confirmed we will inform the donor and provide counselling and referral," he said.
Colorectal cancer in Hong Kong is topped only by lung cancer as a killer.
Lee said its prevalence might be related to the local diet and health culture.
According to Hospital Authority data there were more than 4,000 new cases in 2010, with more than 1,800 deaths.