A Hong Kong school was closed on Friday after a dead bird found in the southern Chinese city was tested positive for the deadly H5 strain of the bird flu virus, health officials said.

The closure came after the school clerk, a 48-year-old woman, picked up the sick black-headed gull at the school on Tuesday, which died the next day and was tested positive for the H5 strain, a health department spokesman told AFP.

She picked up the bird -- a common winter visitor -- without any protection and developed a fever, sore throat and diarrhoea but has tested negative for Influenza A (H5), a variant of bird flu.

"She has been cleared in the medical results we received today but tests on her 11-year-old son are still ongoing, with results expected to be released later Friday," said the spokesman.

The secondary school in the rural New Territories, close to the border with mainland China, is closed for an unspecified time for disinfection and while the department traces people who may have had contact with the bird.

Hong Kong was the site of the world's first major outbreak of bird flu among humans in 1997, when six people died from a mutated form of the virus, which is normally confined to poultry. Millions of birds were then culled.

A 59-year-old woman tested positive for bird flu in 2010 in Hong Kong's first human case of the illness since 2003.

The city is particularly nervous about infectious diseases after an outbreak of deadly respiratory disease SARS in 2003 killed 300 people in Hong Kong and a further 500 worldwide.