WASHINGTON - THE H5N1 bird flu virus may sometimes stick to surfaces or get kicked up in fertiliser dust to infect people, according to a World Health Organisation report.

The WHO team reviewed all known human cases of avian influenza, which has infected 350 people in 14 countries and killed 217 of them since 2003, and found that 25 per cent of cases have no explanation.

Most are passed directly from bird to people, they noted in their report, published in the New England Journal of Medicine on Wednesday.

'In one quarter or more of patients with influenza A (H5N1) virus infection, the source of exposure is unclear, and environment-to-human transmission remains possible,' the researchers, led by WHO's Dr Frederick Hayden, wrote.

'For some patients, the only identified risk factor was visiting a live-poultry market.'

It could be that small particles of virus-contaminated fluid stuck to surfaces, they said. Or perhaps fertiliser made from infected bird faeces somehow carried the virus into people's noses or mouths.

'It is unknown whether influenza A (H5N1) virus infection can begin in the human gastrointestinal tract,' they wrote.

'In several patients, diarrhoeal disease preceded respiratory symptoms, and virus has been detected in faeces.'

Government and health officials have stressed that well-cooked chicken cannot infect people.

'Drinking potable water and eating properly cooked foods are not considered to be risk factors, but ingestion of virus-contaminated products or swimming or bathing in virus-contaminated water might pose a risk,' the WHO team of bird flu experts noted. -- REUTERS