Strains known as H1 and H3 are the most common, and are especially efficient in attacking cells in the upper reaches of the respiratory system. Variants of the H5 virus, by contrast, usually remain confined to wild or domesticated fowl. But when they do infect humans it is often with lethal results, as immune systems are unable to recognise and counter the novel pathogen. Of 348 confirmed cases of H5N1 avian flu in the last five years, 216 -- more than 60 percent -- have died as a result, according to the World Health Organisation (WHO). What health officials fear most is the emergence of a new H5 strain that can easily "jump" from birds to humans, potentially unleashing a pandemic on the scale of the "Spanish flu" of 1918-19 that killed tens of millions of people. Read more at: http://news.yahoo.com/s/afp/20080106/hl_afp/healthdiseasefluvaccine_080106180829