By Maggie Fox, Health and Science Editor WASHINGTON, Feb 11 (Reuters) - Ninety percent of the people infected with bird flu have been under the age of 40, and 60 percent of them have died, according to the latest analysis from the World Health Organization. But the WHO researchers stressed their analysis did not suggest why this might be and noted there are several theories on why the H5N1 virus seems to attack younger people. The H5N1 avian influenza virus has infected 272 people in 10 countries since it re-emerged in 2003. It has killed 166 of them. It remains mostly a virus of birds and has killed or forced the culling of more than 200 million chickens, ducks, geese, turkeys and other fowl. But its occasional attack on a human being has researchers worried. They fear it could mutate into a form more like seasonal flu, in which case it would be spread around the world by people and could kill millions. So WHO is watching it very carefully and keeping statistics on every case. This kind of work, called epidemiology, can help scientists understand diseases, who they infect and who is most in danger. WHO's Weekly Epidemiological Record, published on its Internet Web site at http://www.who.int, analyzes all laboratory-confirmed human cases of H5N1 infection reported between late November 2003 and late November 2006. The analysis said the median age of people confirmed infected was 18 years old and ranged from 3 months to 75 years. "Just over half of all cases (52 percent or 132 out of 256) were aged under 20 years, and 89 percent were aged under 40 years." This could be because it usually requires intense contact with a sick bird for a person to become infected, and in most countries, it is younger people, often children, who tend to poultry. But there could be other reasons, such as immune system response to infection, scientists say. The WHO researchers found that H5N1 has killed 60 percent of its victims and found big differences in fatality by age. "The highest case fatality rate (76 percent) was found among those aged 10 to 19 years; the lowest case fatality rate (40 percent) was found among those aged over 50 years," the report reads. Bird flu killed 44 percent of victims under the age of 5 and 66 percent of those aged 30 to 39. WHO scientists plan a meeting in Turkey next month to talk about why such differences may exist.
