The H1N1 "swine flu" virus is spreading rapidly worldwide, but scientists have puzzled in recent years over why the H5N1 bird flu thus far has yet to do the same.

Now a new study blames our cold noses. Avian influenza viruses do not thrive in humans because the temperature inside a person's nose is too low, according to research published May 14 in the journal PLoS Pathogens.

Meanwhile here are the latest H5N1 bird flu stats from the World Health Organization: human cases have been reported in six countries, most of which are in Asia — Cambodia, China, Indonesia, Thailand, Turkey, and Vietnam. There are 424 lab-confirmed cases of bird flu, with 261 deaths worldwide. The first patients in the current outbreak, which were reported from Vietnam, developed symptoms in December 2003 but were not confirmed as H5N1 infection until early 2004.