Scientists in the United States say they believe they have discovered what makes bird flu viruses so deadly to humans.
Researchers at St. Jude Children's Research Hospital in Memphis, Tennessee, say the culprit may be a protein found in the genes of many avian flu viruses. The scientists say this protein can attach to proteins in human cells.
They found the protein only in the avian flu viruses they sampled, and not in any of what are known as the human strains of the disease.
The researchers say this striking difference between the two viruses may explain why avian influenza is more lethal to people than human influenza.
The findings are published in the journal Science.
Avian flu is blamed for the global pandemic of 1918, but not milder outbreaks later in the century. Bird flu has killed at least 81 people in East Asia and Turkey since 2003.
Some information for this story provided by Reuters.