According to the Centers for Disease Control and Prevention (CDC) Avian Flu in Birds: Avian influenza is an infectious disease of birds caused by type A strains of the influenza virus. The disease...
From other birds since it is a flu virus. It moves among birds just like the regular flu moves among people. By direct or indirect contact between the birds. Birds spread the virus in their feces and...
Any bird could be, but most that have been infected are wild, such as wild ducks and other water fowl in the wild, or among flocks of other wild birds rather than domesticated birds.