The causative organism of Avian influenza is a virus. Avian influenza, also known as bird flu, is a highly infectious viral disease that affects birds, particularly poultry. The virus responsible for avian influenza is primarily transmitted among birds through their respiratory and digestive systems. The virus can also be transmitted to humans, and in rare cases, it can cause serious illness or even death. It is important to note that avian influenza is not caused by a bacterium, fungus, or protozoan, but by a specific type of virus called the avian influenza virus.