There are many avian influenza or "bird flu" viruses and they occur naturally and commonly among domestic poultry and some wild birds, especially waterfowl and shorebirds.
Avian flu viruses mainly infect birds. In low pathogenic strains, disease is mild or absent. In high pathogenic strains, mortality in commercial poultry flocks can be up to 100 percent.
Only in rare instances are these viruses passed to other animals and people — usually through bodily fluids from infected birds.