Avian influenza is caused by influenza viruses that mainly
affect birds and poultry. People may be infected with avian influenza
viruses (mainly influenza A H5N1 and H9N2 viruses) through close
contact with live or dead infected birds and poultry or their droppings.
Among the different groups of avian influenza viruses, H5N1 is of
particular concern because it has a recognized ability to pass directly
from birds to humans and once in humans, the virus may cause severe
disease with high mortality. Nonetheless, human-to-human transmission
of the avian influenza viruses is inefficient.