WASHINGTON (Reuters) - The H5N1 bird flu virus maysometimes stick to surfaces or get kicked up in fertilizer dustto infect people, according to a World Health Organizationreport published on Wednesday.

The WHO team reviewed all known human cases of avianinfluenza, which has infected 350 people in 14 countries andkilled 217 of them since 2003, and found that 25 percent ofcases have no explanation.

Most are passed directly from bird to people, they noted intheir report, published in the New England Journal of Medicine.And very rarely one person can infect another -- always closerelatives via intimate physical contact.

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

"For some patients, the only identified risk factor wasvisiting a live-poultry market."

It could be that small particles of virus-contaminatedfluid stuck to surfaces, they said. Or perhaps fertilizer madefrom infected bird feces somehow carried the virus intopeople's noses or mouths.

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

"In several patients, diarrheal disease precededrespiratory symptoms, and virus has been detected in feces."

Government and health officials have stressed thatwell-cooked chicken cannot infect people. "Drinking potablewater and eating properly cooked foods are not considered to berisk factors, but ingestion of virus-contaminated products orswimming or bathing in virus-contaminated water might pose arisk," the WHO team of bird flu experts noted.

ENDEMIC IN BIRDS

H5N1 is considered entrenched in parts of Asia, includingIndonesia, Africa and the Middle East. It pops up frequently inEurope and has prompted the slaughter of hundreds of millionsof chickens.

The researchers noted that people only rarely becomeinfected. The fear is that the virus will mutate into a strainthat passes easily from one person to another, setting off apandemic that could kill millions of people in the space of afew months.

"After exposure to infected poultry, the incubation periodgenerally appears to be 7 days or less, and in many cases thisperiod is 2 to 5 days," the WHO team wrote.

"In clusters in which limited, human-to-human transmissionhas probably occurred, the incubation period appears to beapproximately 3 to 5 days, although in one cluster it wasestimated to be 8 to 9 days."

It usually causes severe pneumonia and tests suggest thatit rarely or never infects people without causing symptoms.

Avian flu kills on average within nine to 10 days and haskilled 61 percent of victims.

Quick use of antiviral drugs can save lives, they noted,although some strains of the virus are more treatable thanothers with Tamiflu, the drug of choice to treat influenza. Itis made by Roche Holdings AG and Gilead Sciences under thegeneric name oseltamivir.