A type of influenza first observed in Asia; also called bird flu. Avian flu is known as the strain of influenza caused by the H5N1 virus. Initially, avian flu was transmitted from bird to bird, especially chickens, then to other animals. Then cases were recorded of animal to human transmission, with deaths from avian flu reported in Asia and Europe. Doctors and public health experts have been concerned that if the transmission mechanism changes so that humans can transmit the virus to other humans, a worldwide pandemic (large epidemic) could ensue.